Overlap in Social Networks

I have been having some casual conversations with a friend at a US non profit who is looking to attract more younger constituents (more on that later.) One of the things that we discussed was which social network(s) should they experiment with? They are seeking to attract people 18-25.

This chart from the folks at compete gives some food for thought. It does not compare demographics, but when you think about Facebook and MySpace it offers some insights.

Alex, in the blog post, noted:

We can see the social graph, as it is online today, consists of mainly personal relationships, though a large group of users belong exclusively to professional social networks and many belong to both. A collective solution to bringing the entire social graph online might do well to take a closer look at the particular needs of this early adoption crossover group.

If you were starting a new social network test, would you think about this early adoption crossover group? How would you reach them?