One of the librarians on the Usability4lib List was concerned about the amount of investment -- particularly in time -- that embarking on a social networking strategy might require for her library.
Well, for starters I wouldn't do it simply for the hell of it -- i.e. simply to say you're doing it.
On the other hand, I don't see why it should require an inordinate amount of time, especially if you approach it in small steps.
You say your library is going to do user surveys, and also that your ultimate product will be a CMS-based website.
You could survey the users as to what they use (IM? Facebook, etc.). You could then follow this up by examining a few individuals and their use of these services.
(Personal note: IM's been around for years but it was only when I started looking at how the student workers were using it that I finally got religion.)
On the CMS side, you could use the properties of a CMS system to churn out RSS feeds (such as news, events, classes, etc.). These could be added auto-magically to your Facebook or MySpace Page -- thus lessening potential overhead while still maintaining currency (it's the Web 2.0 way!)
A great first step is just making sure everyone's comfortable with IM'ing and that they all have screen-names. You might also want to get your own Facebook account to test out the waters.
One way or another, this isn't an 'all or nothing' proposition.