I use LinkedIn for work related "social" networking. The groups serve as a good way to share a professional interest. And the ability to see someone's profile gives you more information about others than a forum or discussion group. Having a picture helps me identify people when I see them in person. Even at large conferences, I've been able to recognize and converse with people because I had "seen" them on LinkedIn. On occasion, people that I'm in a group have send me private messages, either to get help or just to talk more. If I wasn't on LinkedIn and a member of the group, I might not have been able to have those valuable interactions. I agree that LinkedIn has has gotten more commercial over time, which can be frustrating. I find myself spending too much time sorting through some group digest's to get to really useful info. Group moderation can help prevent spamming, though can be a lot of work for the moderator. As for the other social networks - Google+ hasn't seemed to get enough people to join yet. Twitter is (purposely) limited to what you can share. Facebook started as personal/friends network and while they have moved into being more commercial, I still think that's different from a professional network which should be LinkedIn's niche.