Justed wanted to point out that on May we released Gigya Socialize - a social API product with similar value proposition for site owners, but one that has both components level and API level for developers. This provides much greater flexibility for site owners.
For example, sites owners can tag actions on their site and present a news feed so friends can see each other actions actions.
Very true. My take is that new advertising media usually follows the pattern of first attracting some brand advertisers (agencies always like to sell "big ideas" to their clients), but in many cases the big bucks and the scalable model arrives when direct marketeers (which still bring the vast majority of online ad dollars) find a way to make the new media ROI positive.
As Nick said, there is no easy solutions but to work hard to be the first to find the way to combine "social" and "ads" in a way that will work for non brand advertisers. After that you just need to protect this magic model from being copied by Google and you are done :)