They won't get real competition, at least not for a while. Google+ and some others have tried, but they haven't hit the same vibe. You would have to be so similar that it would end up as infringement, I feel.
Truly, the things that really keep FB up there are 1) being able to like and share without actually being on the site (e. g. all those like buttons everywhere, so the site is ubiquitous) and
2) the ability to easily share multimedia content. You put a video on FB and it's just click, share, you're done, and it looks good. Putting a video on A2K, for example, requires knowing how to use youtube code. And even if a youtube button is put into the BBCode Editor, it's still possible to mess it up.
So you get a feeling that FB is everywhere (and it is) and you get very quick positive strokes from sharing.