Sometimes I resist jumping on the bandwagon (arrogance much?) and sometimes I feel like I'm driving the bandwagon. I'm not sure I understand it, but LinkedIn is an online community building tool for fostering professional relationships. The invitation emails have very quickly made their little viral way through a large portion of my coworkers.
My personal opinion is that professional networking is better and of higher quality when it's actually done the old-fashioned way with business cards and hand-shaking, but perhaps I'm being a square.