The CPM will increase based on a number of metrics.
1-They will charge what the market allows. Meaning, they will get what they can. As advertisers buy into this medium, the CPM could go higher, but not necessarily across the board.
If I were pricing, I would select industries and charge different CPM by industry.
The more competitive the industry, the more I would charge for unique page visits.
2-I would design rankings for FB users. Higher "power rankings", charge the advertiser more.
Meaning, some users have more followers than others. And some users will eventually be measured by how much THEY buy and influence others.
I don't want to give away all my "knowledge" but they have a ways to go.
When they eventually get their behavioral metrics working,it will be a gold mine.