It seems to me that the use of social network sites is accelerating. It's now moving beyond the first circle of early adopters who are specifically interested in the social software meme. An example, I just recently received a LinkedIn invite from Rudy Limeback.
I'm registered at LinkedIn and have noticed that activity seems to be taking off. You might want to get in as it seems to be nearing "critical mass"...
Read on for more thoughts on this out-of-the-blue invite
The interesting thing is, I don't really "know" Rudy. He was one of the very knowledgeable posters on Evolt, a web development and design list, of which I was also a part.
In some ways, I consider this spam. I don't recall having had personal contact with Rudy on anything that wasn't list related. On the other hand, I always really respected his answers and definitely acknowledge him as a master of his craft (SQL wizardry and DB design in general). Since LinkedIn is my "connect to everyone" social network site (as opposed to Tribe.net, where I only link to people I've met), I did link to Rudy.
So is being part of one social networking site or another becoming a pre-requisite for anyone who is an Internet professional? Simon Willison is someone else that I recently noticed commenting that he had joined LinkedIn (and who I also become aware of through Evolt).
If you are a professional in any industry (consultant, lawyer, accountant, etc.) it's probably a good idea to start a blog or at least a profile that people can link to. This is a good way of ensuring that any searches on your name actually come up with good, relevant links to you at the top. Putting your own content on the web is the best way to solidify your (digital) identity.