A Facebook mobile ad network would be huge. Mobile publishers are already suffering with low eCPMs, so they are hungry for new solutions. Showing brands/banners is one thing, but showing mobile app ads is another.

Since a lot of mobile apps have some type of Facebook integration (ie: a Facebook app id), Facebook has a good idea of who has what apps installed and they can serve up apps that are relevant to the user both on the interest graph and the data of what other mobile apps they have installed.

Wouldn't be surprised to see them move into mobile advertising hard in 2012/2013.

* Disclaimer: This is all speculation, no insider information was divulged