The answer to the question of networking site is “yes.” Libraries and librarians should use them all. Maintain a presence on the niche networking sites, for they serve their purpose. These include the LIS New Professionals Network, Librarians as Teachers Network, the Chartered Institute of Library and Information Professionals, the American Library Association, and others. Some of these will prove useful, and some will not. This is a distinction which can only be made through experience. Make sure the cost is justified by the return. Sometimes the cost will be literal fees, other times the cost is simply time and effort.
When it comes to the big networking sites, use them for networking, too, but use them differently. LinkedIn is more for personal use. Maintain a presence here and manage your reputation scrupulously. Use this the same way you would use real life networking connections, as a foot-in-the-door introduction. Remember that your reputation is at stake every time you endorse someone, the same way that your reputation reflects on theirs. LinkedIn can be used for professional purposes the same way that personal relationships can be exploited for professional means, so use it to keep in touch with people you might be able to give assistance to or receive it from.
Facebook is a little trickier. Because is often personal, keep your professional use far away from your personal use. Libraries should use Facebook as an entity and never as an individual. For example, never should a librarian post on the library’s Facebook and item that does not serve the needs of the library. Nor should you act as an advocate or agent of your library on your personal page because everything else you post is there too. You would not go to a professional meeting as a representative of your library in anything less that professional garb, so your bathing suit pictures should not appear alongside an announcement for family story time.