The most important social networks in the world are: Twitter, Facebook, LinkedIn, Pinterest, Stumbleupon, Delicious, Tumblr, FriendFeed, Plurk, Reddit, and a few others. If you're sharing a link on that kind of social networks (these are PR10 - PR7 domains), then search engines will start offering better positions on their results.
Me in Practice
I prefere to use just PR9 Social Signals, sometimes I extend the list to PR8 social networks (usually I don't have so much time). Also, I used just Twitter, Facebook, LinkedIn, Pinterest and Google+ to rank my SEO freelancer profile in Google's 1st page, for keywords like NY SEO services.via Different People
Different people means "different profiles". Don't move your thoughts to software like Ultimate Demon, Senuke Xcr, Magic Submitter, GSA SER, TweetAdder, etc. Anyway, these can't successfully use social websites like Twitter, Facebook, Google+ or Pinterest, becasue any suspiciouse activity, needs additional human specific actions, like PVAs, captchas, password resets, etc.When I write "different profiles", I refere to profiles managed by real people (even a person have 3-5 different profiles on a social network). Also, profiles managed by real people are usually linked: Twitter to Facebook and vice versa, Google+ to Twitter, LinkedIn, Facebook, Pinterest, and what social network you like; Pinterest to Twitter, Facebook and Google+, and so on.
How Many Social Signals?
The answer is: how much you can to get. Try to get involved as many people you can. Of course, is very important to get shares via old profiles, also that have many followers (tens, hundreds or thousands).Also, it's the classic way to virality.
