My usage patterns are quite different from yours. Facebook is my primary service. I don't post on Twitter at all, and I rarely even check it, but sometimes I'll retweet something. I have friends that use twitter a ton, but they're actually the few power users. The majority of my friends either don't have a twitter or never post at all.
I also use Instagram very sparingly. I have an account but that was just to see what the hype was all about. I've posted a couple of pictures but a majority of my pictures and albums go on Facebook, where I can tag my friends, and have the vast amount of my friends see it in their feeds. A lot of my friend do have Instagram account but either they use it a lot or they don't use it at all, so again, it varies between those extremes. Those that post their pictures to Instagram, also post their pictures to Facebook, and since Facebook owns Instagram, I feel like it's a win-win for them (driving traffic in both places). They also recently blocked out twitter, so there's that too.
I actually think people are using Facebook more than ever from what I've seen. It's trying to do everything, but at the same time, it's doing everything well. It's powerful, and all your friends are on it.
The iOS app used to suck but it's actually one of the best apps I've seen now. It's super duper fast and they are continually adding functionality to it. Combining the FB and Messenger apps was such an awesome change and I haven't heard people complain about the iOS app ever since the redesign. I still hear complaints about the Android one though, and they definitely are working on that.
So yeah, I think different types of people use these platforms in different ways, but at the core they're all using Facebook, and also Facebook Connect, so I don't think FB is having an identity crisis at all.
EDIT: Another point I'd like to make is that above I pointed out how a lot of users use services like Instagram or Twitter in an "All or Nothing" manner. However I think we can agree that people on Facebook use it on a much wider scale. However,the important thing is, you don't have clustering at the extremes. More importantly, you don't have the clustering at the "Nothing" end. There are some people who don't have Facebook, but those that do, tend to use it in SOME manner in varying degrees which works out to FB's advantage.