I have both Facebook and MySpace profiles. I use both to stay in contact with people that I know and I also use MySpace also to promote roller derby via bulletins, messages and networking.
In the past, I've considered MySpace more of the party site, and thought Facebook to be the "clean" version of Facebook, because using it, you were less likely to get viruses than on MySpace. With all the new ads and apps on Facebook, that may not be so. I use MySpace, and I do not intentionally visit bad links or click on suspicious-looking links there, but I still get viruses from MySpace.
I could see where this would be a problem for libraries and lead to them blocking MySpace. In addition, MySpace is a big bandwidth sucker, and could detract from more "worthy" web use.
The Myspace and Teens Library Success page looked to list about 90 library MySpace profiles, but hadn't been updated since May 2008. When I searched MySpace for "libraries," I found about 500 related items. Not all of these are actual libraries, but many of them appear to be.
Libraries can and do promote themselves the same way. Often, if I am visiting a homepage of someone who's work or writing I appreciate, I'll find out they have a fanpage on facebook, or a MySpace profile. If they have either of these, I'll add them, so I can keep up on them. It's a good way for me to stay updated on artists and organizations I like, and when I add groups or pages now, my facebook friend's see it, and they find out about the page and get the chance to add them too. It's free potential exponential advertising for an organization or persons' work or services. There's no reason for a library service NOT to have a facebook or MySpace page.
While doing this Thing, I searched "Oklahoma" + "library" on facebook and found that the Oklahoma Library Association has a page, and I added them and became a fan. My library-inclined friends (and all the others) will see this the next time they check facebook, and they will know about OLA and have the opportunity to fan the page also. Free publicity, just like that. Only drawback - the virus issue.