I'm on there (as you know, since you just added me). I like the interface design and the way it handles friending/following people.
I'd happily ditch Facebook in favour of Google+ -- I never really liked Facebook, anyway -- but my non-techy friends haven't shown an interest in it. So just on that, I'd say it's got a way to go before it could be a Facebook "killer." I think it'll do well as "another" major social platform, though -- coexisting alongside Facebook, Twitter, and Tumblr.