> I check in once every few months, usually when I need to sign in because someone messages me there instead of email.

From what I've been hearing from internal teams, Facebook is hoping to take some of email's pie. I expect to see them pushing more people to use Facebook Messenger in place of email. And they're working on new types of ads that interface more with the Messenger. E.g. start a conversation with a company instead of taking you to their web site after clicking the ad.