This is anecdotal, but everyone in my social circle is considering quitting facebook. They stick around for the same reason I am, because events are planned on facebook, because everyone is there.
No one really uses the wall though, and messenger completely died when it started forcing you to use it on mobile and people simply started using other message services because half their friends wouldn't respond until they were at computers.
That's not to say facebook the company is struggling. The wall has moved to instagram and a lot of people obviously use watsapp, but facebook itself is almost exclusively a better meetup.
Maybe this is different in the US, and it's certainly anecdotal, but it's the story I hear everywhere.
The user base is big, like you've said, and that means facebook has better potential than almost everyone else, I'll agree with you there, at least in theory. I just don't think Facebook is really a company with enough vision to disrupt online dating. If they were, they probably wouldn't still be trying to push facebook as their core product, and certainly not at the expense of their new apps that people actually like. I mean, it feels like the world is moving on, and Facebook isn't realising it, maybe because of their company name, but it just feels like they don't have the or any vision anymore.