Ad hoc queries
Social applications can go from 0 users to a million users in a very short time. The best DB fit for these kinds of applications is the DB that can easily scale horizontally which is where MongoDB comes into the picture.
Ads can be saved and updated from all kinds of sources. Data is not really related to each other but there is a lot of them.
Archiving a lot of data that may or may not be retrieved often can be done with a NoSQL DB like MongoDB.