My experience with social networks isn't all that extensive. I know the basics of Facebook and could probably teach someone new how to use it but I would never call myself knowledgeable. Twitter, on the other hand, I just subscribed to for this class. I know next to nothing and have only dabbled in it since signing up. I'm fairly certain that if you asked me to teach someone about it it would resemble the blind leading the blind. Pinterest, however, is something that I'm on daily and could be called a bit of an addict. I could probably give a good lesson on this site and sound like I know what I'm talking about. I know there are hundreds more social networks but these three are the ones that I've had interactions with that are worth mentioning. The others I've either never heard of or have never worked with them.
In comparing Facebook and Pinterest, I would say it really depends on what you are receptive to. If you are more of text and reading person, Facebook is the place for you. The postings almost always include some kind of text with whatever is being posted (if it isn't just simply a text post). Pinterest is more for those visually oriented as it is always a picture that can have text along with it. For libraries using these sites, I see them using them as more advertisement and getting information to their patrons. With Facebook or Pinterest, the patrons can subscribe to the libraries personal profile and receive postings. This can be used to promote special events and updates on what's going on at the library. With either Facebook or Pinterest the library could also share postings/pins from followers that they would like to point their patrons to. For example, the IUPUI School Facebook recently made a post telling its followers how the Children's Museum lets college students in for free on Sundays.
While I know Facebook is fairly popular right now with getting information out, I wouldn't overlook the possibility of using Pinterest. It's not as popular right now but it is rising in notability, and it could possibly reach more people. Postings on Facebook are usually only seen by followers and then shared to outsiders. With Pinterest, there is a greater possibility of being seen by those that don't directly follow you. Advertisement for the library could be pulled with searches for books, libraries, or reading and even projects if you post those on the site. All in all, both sites would be a great addition to the advertising of any library.