It may not die out, but the reasons I think it will:
Its gotten too big, and over marketed. Facebook was a lovely thing back when I could keep in touch with my friends, and not have to worry about ridiculous ads and invites to ridiculous groups and games. The feel of the site was much more mature than that of Myspace, which I think is horrible (at least the actual profiles, because I could care less what your favorite song is, I don't want it to be streaming automatically overtop of music I am already listening to. Nor do I want to see your red text on your neon green background).
I've seen plenty of people leave Facebook for several reasons, it's gotten to the point where family members add their relatives, which sounds like a good thing but if you're young, you generally do things and talk about things that you don't want your relatives to see/know. While I don't post pictures of myself drunk or going-out at the bars like so many do, its gotten to the point where people have up and left. They don't want Mom and Dad to know that all that money they spent on college is going to partying!
Where can it go from here? What direction can it go in? I'm not a fan of Myspace at all, but what they have that Facebook doesn't is real, actual content. (Music and videos) Myspace does things to support up and coming musicians, actors, comedians, etc. Facebook just promotes being lazy and playing Farmville and annoying the hell out of me with your invites and a cluttered wall of no friend comments, purely ridiculousness.
Its early, I need to have my morning smoke and coffee and maybe my mood will be better
I do think Facebook will slowly begin to die out, I hear Myspace is launching new features that will be implemented slowly, they want to double their users from their current 120million (I think) to 300million. Or atleast according to a news story I heard about an hour ago on the local news. Coffee and cig time!
EDIT: Mmm, coffee, smoke and breakfast. To the original poster, I do not mean to discourage you at all, that is not what I am doing. I wish you much luck in your endeavor and if you create anything that could directly compete with the major social networking sites I would be happy to join and try it out. I think there is room for one, but perhaps it needs to be less widely targeted and more directly targeted towards either a specific region, group, or interests so it actually provides the service it was created for.