The problem with the emergence of these social networks is that you can get lost. In your communication strategy you will start by defining who you are going to connect, meaning you are targeting an audience more or less accurate.
In this work of defining your target you should also think about the tool you will use. All networks are not addressed to people and even the reverse is also true: not everybody uses the same networks. The choice of communication tools is based on your target.
Facebook is the leader in terms of members as far exceeds all other networks with its millions of members. So you have to do a wide and diverse audience which includes all occupational groups.
DeviantArt, a growing community is a network designed for professionals from designing, application development but has far fewer members still. However, these members are encouraged to be more relevant targets because they are present from a professional viewpoint.
LinkedIn is also referred to a professional network, it is also the No. 1 business network in the U.S. and has more members than others.
Twitter, my guilty pleasure, which is still (too) unknown to the general public but which nevertheless beginning to make themselves known. Its use is rather different: many use to say what they are doing (like a facebook status) but increasingly use them to share information relevant and serious.
We give importance to two networks : Twitter and DeviantArt. Facebook visitors are not sticky, at least from our point of view. We get visitors from all three networks: Twitter, deviantArt and Facebook after a post, but in site stay time is less for a Facebook visitor.
Money matters. DeviantArt visitors click advertisements; for other networks? We get almost nil.