1. Up until a few weeks (months) ago Facebook's iPhone app was complete shit. My younger highschool brother (and all his friends) who are not techies in any sense, always complained about how it sucks. One time I even asked them why they switched to Instagram, and they told me the app works much better than the Facebook app and it's a lot more simple to just post pics/browse. That brings me to my next point.
2. Facebook has grown to the point where it doesn't know what it wants to be. Kind of like myspace was trying to do everything, Facebook is in a similar position (except their products actually work). Instagram specializes in just photos, and makes it easy to use. There is no other crap to get in the way, you upload pics, like them, and comment on them. That's it. Twitter specializes in just short, to-the-point comments. Where does that leave Facebook?
3. Facebook is establishing itself as a kind of social media aggregate. When I post anything to twitter, instagram, or tumblr, it automatically gets posted to my Facebook. But I rarely ever post anything directly on Facebook. From my experience, nobody is leaving Facebook, they just use it less. And as a side not, I think Facebook's strongest features are Groups and Events, I use those two features more than anything else. It is so easy to create groups and invite people, it's awesome.
4. Instagram has cool photo filters that make it very easy to make your photos look "cool" and cover up any imperfections in the pictures. Take any shitty picture and apply a filter, all of a sudden it doesn't look too bad. Sure it looks generic, but it doesn't look like complete crap (most of the time).
Anyways, that's my take on fb/twitter/instagram.