I think one of the biggest advantages to sites like facebook is using them to network with people in the same industry as you that you may not normally have a chance to interact with.
For example, for a musician facebook is an invaluable tool in that I can add a person to my facebook friends list that I met at an industry party or an event that I may want to use in the future to further my career.
To take a step further, say I play the CCMA's (Canadian Country Music Awards) with a band. At the same showcase that I'm on, there are several other artists that are using a house band.
The bass player for the house band is a heavy hitter who plays with a major label artist/star.
Said bass player likes my playing and we chat/exchange info/add each other on facebook.
We keep in touch over the months following the CCMA's and as the next years CCMA's approaches, awesome bass player finds out that he/she can't do the house band gig, but because we've kept in touch and bass player knows my playing, he/she asks if I would like to sub for them on the gig.
All of a sudden using facebook has payed off in that I now get to do a high profile gig/get a decent paycheck and the opportunity to step my career up a notch.
While I have never been part of the house band at the CCMA's, I have absolutely gotten hired for gigs because of exactly this scenario.
For this reason alone I feel that it is totally necessary to maintain a social networking site profile.
Also, this is only my opinion, YMMV, whatever, etc...
