Google+. It has a legitimate chance to be really good. But like any other budding social network, a good app is a requirement. Google+ for Android looks to combine features I use from other apps into one single portal. Awesome.
The home page of Google+ acts as launcher for the four main features of the app: Circles, Stream, Huddle and Photos. Circles lets you share things within a select group of people. Stream gives you a live timeline of updates from your friends, not unlike what Facebook's news feed does. Huddle is like GroupMe or any group chat service, where you can create groups on the fly and talk amongst yourselves to plan out a night, a holiday, etc. Photos is really great as it has an 'Instant Upload' feature that can automatically send your pictures and videos to the cloud for super easy sharing.
It all ties together for a promising one-stop shop to stay close with my friends. I think Google+ has a really good chance to mix the perfect combination of features I use to connect and share things with my friends - if enough people get on, it could theoretically replace everything else. You have to have a Google+ for the app to work and it's only currently on Android. An iOS version is supposedly coming (though a web app does exist). [Android Market]
Check out Google's video on how Google+ works on Android here: