http://finance.yahoo.com/news/facebook-seeks-breakthrough-mobile-ad-141949912.html

excerpts:

"Facebook is now planning to use its knowledge of its members, gathered through their voluntary sharing of status reports, personal details and likes, to build an offering to rival Google's."

...

"Users may also perceive advertising that is too personalised as creepier on such a personal device as a phone in their pocket that knows where they are than on a desktop."