I never used Friendster and I used Myspace and Facebook very little before I decided that it just wasn't for me. I'm not sure why Friendster didn't improve and I think that Myspace is just a coding mess. It very well could be an ego thing, that's good some times to stand by what you think but if your users want something and you are unable or unwilling to provide it for them then you can't be surprised when they jump ship over to the new thing. It may not actually be related to what site provides which features at all, it simply could be people wanting to try something new.

As for who will dethrone Facebook, I can't say. I'm really not into the whole social networking thing so I don't know what to look for. It sounds to me that you have an interest in it. Why don't you give it a shot? I think one thing that would set you apart is letting your users easily customize the layout of their profiles. You can do it in Myspace but it's a nasty non innovative process and I don't think you can do it in Facebook (I could be wrong). Unless there is some nasty security problems that I can't think of I really don't see the problem with allowing your users to have a little CSS freedom. Maybe you could come up with an editor of sorts to allow people who aren't savvy with CSS to be able to customize the look of things.

If you do try this, I hereby request that you call it Myfacester. ;)