Facebook is the new crack for sociable people and really can’t be entirely ignored if you are trying to build communities–a prerequisite for any modest success for a language of any sort.
It is easier to make friends of people who are interested in conlangs than to make friends become interested in conlangs. For any given conlang, the interested audience will be evenly distributed across the entire world.
The above facts work against using Facebook as a conlang community building tool.
Facebook figures you know everyone in person, in real life
People in conlang communities tend to take handles, don’t necessarily post their picture, their real email, etc. So except for certain people, it can be hard to find anyone on Facebook.
Facebook, assuming you all know each other in real life, assumes that everyone you know in real life is much the same. In the case of in town friends, office friends and family, this is probably true. We all speak English and have a passing interest in our friends and friends of friends. It isn’t true that all of these people are equally interested in conlangs or having conlang related messages appear in their news stream– or worse, messages written in a conlang.
Worse if you try to segregate your audiences by creating two accounts, Facebook doesn’t support a wide variety of clients like Twitter does, so there isn’t an obvious way to manage two Facebook accounts other than opening two browsers.
Groups/Group Pages/Company Pages/”Like” Pages
Facebook keeps introducing these peripheral places to post messages. Except for Groups (which appears to be an ordinary mailing list) the others give you too little notification about new messages for them to be effective ways to communicate–unless you visit each of those pages every day. Come on! Are we back in the pre-rss internet days? When there really wasn’t a way to unobtrusively find out when a page has been updated?