Seems like it will take off because it looks to be pretty integrated with services that are already pretty popular (gmail, mostly). I wouldn't say it's trying to replace Facebook itself, rather become the default microblogging/status service, the trend that Twitter is famous for and Facebook is trying to also claim.

I'm calling it the unproductive Wave. This is a really awkward time for social media. It's hit puberty and is growing in ways that it doesn't understand and won't until it can look back in a few years and go, "that was weird." And in the meantime, you're going to have relationships with the awkward teen. You'll break up, go out with another one and eventually settle on the one you like best. Until college, then you'll break up and never see each other again until the 20 year reunion.