For the interested social researcher:
I let xing.com check if people from my desktop address book are on xing, and if I am connected with them (a feature that linkedin had before them, I think).

• 623 contacts in my address book have email addresses and can be compared
• 105 (17%) of them are Xing members
• I am already connected with 52 (8%) of them

Does this say anything scientifically? Hm....

and, you 53 people, expect to get mass-mailed soon...