There's been much written about Facebooks ad announcement. Erick Schonfeld did a nice job livebloging from the event. It will be interesting to watch how this plays out... The obvious risk is that users are driven away by in-your-face advertising with the inevitable 60pt blinking text, auto-play audio clips, etc.
More likely though, I see legions of brand loyal consumers propagating the message to their finely targeted prospects, err, I mean friends. I spent some time with Greg Selkoe, creator of Karmaloop.com some time back. These folks had built a small army of brand supporters - a team of people who would promote through email, IM, MySpace, etc;. In return, Karmaloop offered certain percs - be it featured designs, discounts, visibility through the eCom storefront, etc;. It was masterful... By far the most effective social advertising I'd seen to date.
I believe similar will play out on Facebook. Look at some of the partners who have already signed up - Coca-Cola, Verizon, Sony, Blockbuster, CBS... I see Coke (as example) offering percs to consumers who rally around their brand. Perhaps product discounts, special 'insider' widgets, cool 'members only' avatars, etc; - And all the user has to do to take advantage is acknowledge their support of the Coca-Cola brand. It's word of mouth / peer pressure advertising at it's best. Equally as interesting will be watching how far companies are willing to e-recruit their legions of free advertisers.
The game changes, yet again.