I use Google+ communities myself and there are a wide number of professionals who are happy to knowledge share and support each other and myself and many of my peers, will freely provide valuable consultancy advice to community members. Yes we are actively demonstrating expertise which I guess helps with reputation but mainly, we help others because we are passionate, friendly and value the communities we are a part of.
It is good to hear that this sort of community sharing is filtering through and actively being encouraged by social networks like Scrt.ly, Whisper, and PostSecret. Too often peoples personal social media accounts are carefully edited versions of their real lives which present a skewed, often idealised image that many others will find difficult to live up to. The honesty and sense of caring community offered by this new breed of social network sites is in my opinion a very welcome step.
I do however have a real problem with totally anonymous social networks (lets take 4chan as an example). I don't believe anonymity encourages honesty or a sense of community. In my experience it encourages hostility and a total lack of morality as normal boundaries are removed, and these social networks as a result, attract a very particular type of person who I personally really wouldn't want to take advice from or reveal my problems to.
I wonder why people feel the need to post anonymously about their problems? For me this shows the somewhat vacuous nature of the likes of Facebook where many people are 'friends' with people they only want to impress or prove something to; if you have 500 friends, but wouldn't share problems with them, they clearly aren't friends at all. In the race for popularity "Look how many 'friends' I have!" people have forgotten that real friendship and a real community is supporting through both good and bad. If people were able to maintain their own authenticity on social media there would be no need for anonymous social sharing sites at all.