You can target a group of specific users. There are some restrictions (minimum group size) but there are ways around it.[1]

[1] http://mysocialsherpa.com/the-ultimate-retaliation-pranking-...

This is fucking terrifying.

You could until a couple of months ago, and you still can with some significant caveats. Also, combining names and locations with other data from third parties gives some pretty awesome postal mail/email/telemarketing lists for companies in this space.