My understanding is that Rovio is using the location data it obtains when you play Angry Birds along with the time you are playing Angry Birds and is monetizing that as information regarding the location of restrooms as well as the general conditions in that restroom.
That information is very valuable to the right vendors when seeking to sell their restroom cleaning services.
I believe it isn't free on the iPhone but is on Android and for Android there are ads all over the place. The advertiser pays to put those ads up.
Angry Birds wasn't free, it cost me $1.19 AUD and I think it's $0.99 USD. The lite version of Angry Birds is free, but you don't get nearly as many levels. Quick fact: Angry Birds cost $140k to develop and has made around $70 million in profits.
I know they make a lot of money via endorsements and whatnot these days as well.