Facebook & Microsoft Deal : Why not a advertising platform for local and mid-size advertisers instead ?

The Facebook platform have been at this place in the top place in the Web since few months in America for two reasons :

1. A developers platform . Great viral effect and massive new applications developed since end of may.
2. The switch of the primary focus clientele from colleges to everyone.

But where is the money ! For now, more than half seem to come from insignificant ads from Microsoft. And now, they will sell 5% to MSFT ?

After seen the possible with MSFT, I was wondering why the Facebook team are so dependant of Microsoft for their revenues ? They got a huge community divide in a lot of cities network with a lot of open profile. Why they not start their own advertising platform instead ?

You got the answer ? Please comment on it down here.