Ofcom have just released a survey in the UK about Social Networking. That might give you some useful data. http://www.ofcom.org.uk/advice/media_literacy/medlitpub/medlitpubrss/socialnetworking/
Have you seen the Pew Internet reports? They're based in the US, and often have information on such things.
I have tried researching your question and it was hard finding answers. I guess there isn’t much research done on this yet, but according to an article written Sep 11, 2007 on www.informationweek.com titled “Do Online Social Networks Lead To More Real Friendships?” the average person has 150 friends on a social network (not sure this accounts for email or just networks such as MySpace and Facebook and others).
I can tell you my numbers; I have 333 email contacts and only one email account and 33 facebook contacts (clearly the number 3 is key today!).
