Unwanted spam from a Social Networking site. legal or not?

[Twinsen]

Hello,

I have a quick question concerning e-mail spam I've recently started receiving from a well-known social networking site. I am getting notifications about people adding me as friend on that networking site even though:

a) I myself do not own an account on said site, nor have I ever owned one
b) I have not given my e-mail address to that site nor signed-up for any sort of notifications.

So far, I've had about 25 of these "friend notifications" arrive to my inbox since the whole thing started about three weeks ago.

I would like to know how is it possible for them to use my e-mail address without my consent, and if such an e-mail practice is illegal (because it certainly seems like harrasement to me,) are there any grounds for suing?

I am not a resident of the U.S. (I live in Europe) but the SN company in question is U.S. based.

Thank you,

Twinsen