It's getting easier and easier for services to raid your address book.
I got nearly a dozen notes from people I don't even know today, all asking me to join their network at a service called Quechup. "Wow," I thought, "this service is really getting traction."
Then I got a note from Scott, pointing out that the service had automatically sent email to his entire address book without his participation. My guess is that it's not quite that automatic, but there's definitely a danger here... a danger to services that end up alienating people by sending email they didn't expect, a danger to people who end up alienating their network, and a danger to my (and your) inbox, which is already overflowing.