I have used Facebook and LinkedIn, but I definitely use the former more than the latter. I use Facebook recreationally, and as a way to keep in touch with family members who live across the country. In the past, I have used LinkedIn for business purposes; connecting with previous employers, and keeping a record of skills I possess for current or future employers.
As a future library and information science professional, I see value in using both of these services. I know as a library patron, I have attended events at my local branch that I have seen advertised on Facebook. Had the library not used Facebook to advertise the events online, I do not believe I would have know about it. Because of instances like this, I believe Facebook is beneficial to the success of libraries. Also, I follow IUPUI on Facebook as I have "liked" the page. I live in St. Louis, and I have never seen the campus in-person, nor have I spent much time in Indianapolis. This weekend, I saw a post about the city being decorated for the holidays, and how they are celebrating in the campus bookstore. Following the page on Facebook is a way for me to stay connected to the campus while living far away.
Using LinkedIn would be valuable for a library professional to stay connected with other libraries, or for use if one is seeking employment.