Fremont-based Offerpal Media, a developer of virtual currency software for social games, said today that it has acquired Tapjoy, a developer of virtual goods and other monetization services for the Apple iPhone. Financial terms of the buy were not disclosed. Offerpal was venture backed by D.E. Shaw Ventures, InterWest Partners and North Bridge Venture Partners. Offerpal said it would use the acquisition to accelerate its expansion in the mobile market.