Actually, it's not spam at all. It's just a system (let's not go down the "evils of giving someone my email user/pass" road again) that allows users to more easily send to
their contacts.
To look at it another way, if you like our widget.com site, we at widget.com are trying to make it as easy as possible to tell your friends. Re-entering your contacts into our system manually is such a pita, that almost no one does it. I don't know about you, but I have never received spam from MySpace, Facebook, LinkedIn, or Yelp. I have received invites from friends/associates/folks I kinda know. This is not spam, this is email from a someone I know using a different sender. Is it always welcome? No. Do I really mind it? No. If you do, you should have a filter on your mail that trashes all mail from addresses other than those in your contact list.
We all know that the centralized contact model with API's and public/private keys would just plain rock, but it's not here yet, and companies are just trying to live with what we have (long enough to get their next round).
Ok, and one more thing - for those of you who try to send emails, this post is a nice summation of the things you should do, have to look out for, should know about, and the sites you need to visit to improve the chances of your legitimate messages getting through.