How is this surprising? We need 'multi-headed' social networking.
Worst thing about those sites is the way they exclude their competitors (or rather they pretend that the competitors don't exist). None of the users benefit from this idiotic policy, which after all is just a dumb strategy tax. For that reason they will never be able to compete with good ol' fashion email & telephone, unless they can come up with a more inclusive business model. It's not platform lock-in, it's platform lock-out!
The killer app would be a social networking site which automatically registers you with [your choice of] the most popular social networking sites, gathers your contacts from all those sites, synchronises the data (so that if you have a friend on more than one site, the profiles get merged), scrapes any updates and presents them for you in a nice, user-friendly way. I know that's a tall order, technically speaking, but in terms of commerce, it seems like a no-brainer, so why has nobody done this yet?
facebook appears to be the most 'open' because it allows you to mash up apps which scrape content from other social networking sites, even lets you link to them directly, but you still have to set up an account with those sites manually. facebook also lets you share your actual email address and phone number, which the others seem to actively discourage.
With messenger clients, it will always make more sense to use (say) Trillian or Adium instead of the locked-in single platform versions. First to market with a 'multiheaded' social network site will score big.
It would be great to be able to pick a social networking site purely on the basis of features, but of course, no matter how great a site is, they insist on resorting to some kind of platform lock-out because they force their users to be fickle.