(I am not ingres even though I am asking a question you asked them.)

I don't like Facebook as a venue for such things because:

• It is a walled garden; in general material on FB is not visible in web searches and can't be linked to directly.
  • I think some categories of public post on FB are linkable and searchable, but I'm not sure exactly what, and I suspect comments on linkable-searchable-things are not themselves linkable and searchable, and (see the next point) I have no reason to think that what's linkable and searchable now will remain so in the future.
• Anything on Facebook could disappear, or become less accessible in some other way, or become surrounded by billions of annoying advertisements, at Facebook's whim, and nothing I know about Facebook makes me think such outcomes are terribly unlikely.
• I have to assume that anything I do on Facebook is being tracked and machine-learning-ified by Facebook. I don't see any super-obvious actual problem with FB knowing that I talk about things with rationalists, but on general principles I want as little as possible to be visible to Facebook.
• I have to assume that anything I do on Facebook is going to be shown to everyone I am "friends" with on Facebook. Again, it's not going to be news to any of them that I talk about things with rationalists, but again I have no particular wish to advertise everything I do to everyone I know.
• Any time I am on Facebook, I am bombarded with social fluff. I value the social fluff a lot (otherwise I wouldn't be using Facebook at all), but I don't want it in my face when I'm trying to have a discussion about AI safety or effective altruism or any of the other at-least-one-notch-more-intellectual things that come up on LW.
• I do not want to contribute to Facebook's increasing domination of the web. I use it for social networking because really there's no alternative, but the less support I can give to the facebookification of the internet the happier I shall be.
• Some people (for reasons resembling the above, or other reasons of their own) don't use Facebook at all, and I think it's very unfortunate for them to be excluded from discussion.