Yes, you can have your own social network baked right in. With lots of features.
And (this is good cause he has a revenue stream), there's addon plugins you can buy e.g. Facebook and Twitter plugins, support for premium members, geolocation, more.
I did think this was gonna be a cake installation
>gem install community_plugin
but 'tis not. ...yet. There's actually a lengthy install process to go by, starting with downloading it from git.
Still, developer Bruno Bornsztein has given us a beautious project site...and that's not nothin either. Of the 18000+ gems available for Ruby developers, precious few have even a recent update history, let alone a fine project site like this one.
A project to keep an eye one.
Here's a couple more found in a quick search: Insoshi and Loved by Les. For Twitter and Facebook specifically, Twitter gem and Rfacebook.