Your friends, family, 'net pals, gaming buddies, and so on.
If you already have a personal social network on these sites, but don't really want to participate on them, but do want watch and keep up-to-date with everyone - thanks to the very web standards most of this junk is built on, you've got a way to do it without being glued to it.
Screen cap of my preferred RSS feed reader, Google Reader:
Just like subscribing to a blog, you can subscribe to individual twitter feeds and even on Facebook, you can subscribe to links, notes, and status updates (with a little bit 'o hackery).
Like I said, the point of this is that perhaps you don't or simply can't log into these sites from work, or just prefer to stalk them, without actually logging in and engaging with them.
The point is you have the ability to receive your friends social network data from outside of the social network. That freedom gives you more flexibility and choice.
(this is a quick and dirty post...I plan to update for clarity, perhaps with more information at a later time.)