I have read some blog posts by other writers about the "questionable" email practises of tagged.com (wherein they login to your email address book and spams everyone in it, encouraging them to sign up for their own page).  I agree with the other writers' conclusions: that's unethical and looks a lot like spamming.

Facebook is a little different.  When you sign up for them, they ask you for your email and password (while promising never to use it for nefarious purposes), check your address book and then send invites to people in there or cross-reference your address book with people in Facebook with that email address.  In other words, with regards to the latter, they hook you up with your friends that are already in there.

Today, I came across this conspiracy theory page on Facebook.  The creator of the Flash animation lists some uses of Facebook including how they collect information on all of their users and how they reserve the right to sell that information to third party vendors.  This got me to thinking: how will it be before spammers start posing as legitimate marketing companies and start paying for live email addresses like what we see in social networking sites like Facebook?  I think it's only a matter of time before someone tries.  While I don't think Facebook would do anything so foolish as to release user-information to a spammer, some other lesser known social networking sites might.  If somebody in their basement is trying to build their own site and doesn't have good security policies, the lure of easy money might be difficult to resist.