Call me crazy, but I prefer it when companies do this. If I'm interested in using their service, then I'd be happy to be alerted when my friends sign up for it.

Call me crazy, but I prefer it when companies do this. If I'm interested in using their service, then I'd be happy to be alerted when my friends sign up for it. That being said, I wholeheartedly agree it should be opt-in (or at least have an opt-out) for people who are concerned about their personal data.