In The Passive Mass Audience Has Left the Building, Dave Evans has some tips about interactive marketing. He writes:
... Marketers are moving into social networks, both as participants and platform owners, because they can market in ways not possible in a broadcast environment. Broadcast can reach lots of people, no doubt about it. But to get a message where it really needs to go: on my to-do list or into my friend's inbox with my name in the sender field, you must be part of the social goings-on. You must also market via a platform that allows consumers to add value to your message, to validate that value through friends' approval or disapproval, and to let you know (as a marketer) what worked and what didn't. You can achieve all this in a social network. ...
And see also this related essay from Gary Stein: Advertising on Social Networks: The Third Wave. Link: ClickZ.
More on this topic from Pete Plackshaw: Meaningful Relationships with Social Networks. Link: ClickZ. --Dennis