Summary
If you come up with interesting insights or detailed arguments in the course of Facebook discussions, you might consider summarizing those ideas and saving them in a standalone article for others to read. This would be a more concise and shareable way to preserve your thoughts than just linking to a long discussion thread.
Contents
Pros and cons of Facebook discussions
Some friends made a comment recently that stuck with me: People spend a lot of time discussing among themselves on Facebook (whether privately or on threads), and often, the ideas aren't preserved or shared with a wider audience. There are two problems here.
Problem A: The information is repeated over and over to the same people, when it could be reaching new people.
This is tricky. It requires sustained time and effort to share a blog post with a wide audience. I often hesitate to start discussions on Facebook or other active forums like LessWrong because I know that if I do, I may need to reply to extensive comments for hours afterward. Maybe I don't actually need to reply to everyone, but I feel bad if I don't; perhaps I should change that instinct, although I also worry about people seeing me as insensitive as a result.
The other reason it can be hard to reach new audiences is that you have to find them. There are ways to do this—e.g., going to new forums, posting on the Facebook group of a related organization that's not already aware of the issues, etc. The point is this takes some effort, although it's often worth it.
One other way to combat Problem A is for someone else to share the post for you. That way, readers will think you may not have seen the comments, which means you're not obliged to reply. For this reason I don't mind at all when people share my posts on my behalf.
Another benefit when others share your posts is that it ensures some degree of peer review. Not every post is useful enough to be shared in the first place. If someone shares it organically, it's more likely worth sharing. A similar goal is accomplished by Likes on Facebook leading to a post being promoted more in news feeds.
This issue arises when Facebook discussions become long, ponderous, and contextually burdened. People may have to read 50 replies to know what's going on. It's hard to link to a Facebook comment and say "this is what I'm trying to say" in a later conversation because the person won't want to read the whole thing. Sometimes Facebook conversations aren't publicly shared. And even when they are, they're not easily searched, and they're not indexed by search engines. If the Internet at large is paper that self-destructs within years, Facebook posts are paper that typically self-destructs within days.
On the other hand, if you want to share something with broad reach, Facebook tends to be the way to go. It's hard to share with a similar number of people elsewhere, and one advantage of Facebook is that posts can be read by friends who aren't already interested in altruism. The Like functionality of Facebook is really awesome for showing and collecting feedback.
Thus, there's this awkward limbo situation where Facebook doesn't serve all of our needs (easy linkability, preservation, searchability), yet we can't do without it either in terms of sharing and collecting feedback. Peter Hurford and I discussed this, and we decided the best strategy for now is to write blog posts somewhere else and then link to them on Facebook. Alas, this leads to a balkanization of comments where some people comment on the original forum and some comment on Facebook. It's a mess. Maybe one day it will be resolved.
Distilling Facebook discussions
In the meanwhile: When you do have conversations on Facebook, it may be a good idea to extract the essential juices from them and convert those into a blog post (or something else linkable without burdensome context, 
like the essays on this website) so that the ideas can be preserved for future sharing rather than getting lost. The same applies for verbal or private textual discussions.
This isn't a hard rule. It's sort of like the question of whether you should take notes at a meeting: Sometimes it makes sense to ensure you keep track of important things and can share them widely, but sometimes it's a waste of time. But it is at least worth thinking about who and how many will read your Facebook comments and whether those comments continue to provide value after the immediate discussion ends.
Finally, I don't want to sound Scrooge-like. Many times Facebook comments are about having fun with friends rather than evangelism or serious conversation. That's fine and good. I'm referring to cases where you expend energy writing a lengthy comment reply that may in fact have limited reach or may be preaching to the choir. I write plenty of those myself, and they're often not a bad use of time. But there remains the question whether you could make even better use of the time by writing similar things in a different place.
A related, hybrid idea that I've started doing is to write a comment on Facebook that's sensitive to the context of the conversation but then link to an external post that contains a more standalone description of my thoughts. The latter is more linkable and preservable. (One problem with this is that people are far less likely to read the contents of a link than they are to read something directly on Facebook. This tends to incentivize excessive copying of text, though maybe that's the best we can do. It would be interesting to learn what fraction of Facebook readers actually click through on links in conversations.)
Second-rate alternatives
Quora, maybe Yahoo! Answers, and forums seem somewhat better than Facebook because at least the information shared is searchable and available publicly. It's not as coherently organized as in a blog post or essay, but it's still more useful than Facebook conversations. I especially like Reddit because it allows for creating "forums" (i.e., subreddits) on lots of topics without the hassle of running lots of forums or having separate passwords for each forum.
David Pearce's data liberation
David Pearce has a web page that collects a vast array of Pearce's comments on Facebook, Quora, Reddit, and other social-media platforms. I admire Pearce's dedication to saving his content, especially content on un-Googleable platforms like Facebook. I have found Pearce's social-media pages useful when trying to Google for things that he has written about.
Personally I prefer to add insights I have to an existing article on one of my websites, so that the new insights can be organized along with other relevant material on the same topic. (For example, in 2018, I added the section you're now reading on "David Pearce's data liberation" to an existing article that I had first written in 2013.) However, it may be that a Pearce-style page for random social-media comments would indeed be helpful as a place to store things that don't really fit anywhere else. And for people who don't already have hundreds of existing articles on their own websites, Pearce's approach of just saving individual comments might reduce the "barrier to entry" for preserving social-media content.
I mostly consider my Facebook comments, emails, and other conversational content to be "throwaway" data because it falls into one of several categories:
- It mainly just links to and perhaps quotes from an existing article. In this case, there's no real need to save the comment, because it duplicates existing info.
- The comment is embedded within a detailed and highly context-specific discussion that I sadly don't have time to reproduce in full on the web.
- The comment is about personal details, internal organizational matters, or transient administrivia.
- The comment is about a topic where I'm uninformed or where I haven't read the relevant article in detail, and I don't have time for the due diligence that I would want to perform before putting the comment on my website.
However, for posts and comments that don't fall into any of these categories, saving the content to your website (and then linking to it or quoting it on Facebook) seems like a good idea.
Acknowledgments
A comment by Adriano Mannino helped to inspire this piece.