I get asked this every day and here is part 1 of a reply to a post I did over on www.xbiz.net
General Social Networks
1) Facebook (1 personal profile & 1 company fan page min).
2) LinkedIn (1 personal profile & 1 company profile and 3-5 industry groups plus 1 proprietary group (optional and only if you maintain it daily).
3) Myspace (yes I said myspace...don't count them out yet now that it sold at market value and managage debt service). (1 personal or company profile)
4) Google+ is the new kid on the block and pretty lame and redundant to me at the offset but gaining in my esteem simply based on the fact that Google owns it and although I think it needs A LOT of improvement they will put the time and money into doing so. (looks to me like they rushed an Alpha product through and are dumb like foxes by letting the early adopters do their dirty work for them). For now...be there but give it only minor attention.
5) Twitter is a must have, still don't get it really, perhaps I'm just too long winded as this post exemplifies but it's for those people who just read the funnys on Sunday in my opinion but the sad part is that's a huge percentage of the population.
6) One Company Blog self hosted stand alone or integrated into main company website. I recommend Wordpress first, Blogger second, Tumblr and Posterous tied in 3rd...leave the rest.
Social Bookmarking Sites
7) Digg.com is my favorite in this category followed closely by Delicious, Stumbleupon, Reddit, Plurk, Diigo, and FriendFeed.
8) Quora & Formspring are the two biggies along with Yahoo Answers and I tip a nod in the direction of Quora for quality of content."
More soon in part II