AO3 makes it very easy to create collections and challenges/gift exchanges, either private or for others to add to, even allows for anonymous posting to certain collections for those who are shy. Also, now that they've decided to allow meta posts, there's opportunity for fandom discussion as well. I think a lot of these features are going unknown so far as it's still in Beta, but it's all there if users explore a bit.

It's not quite the same as LJ, of course, but considering AO3 was made for fans by fans, it's always improving in our favor for fandom. But if there's still fandom here, this comm is still more than available. People just have to post to it.