Unsurprising
I find it hard to believe that anyone would be surprised by those findings for the very fact that they are called 'Social' networks. Why would someone who is social by nature stick to a single site when they can easily keep several updated and widen their social networks a lot more by utilising some or all of the available offerings.
A lot of business people who are on Facebook for example will probably still have a LinkedIn account. Add to that someone who has a specific interest such as music might also have an account with Last.fm. Single people may belong to a dating community but not want to have that associated with their Facebook/MySpace profile.
If a new social network site appears and all your friends start to use it - then you will probably use it too just to keep in touch with them all. However you will probably still keep any old network accounts to keep all those friends that haven't switched.