Pasadena-based PostUp, the Twitter advertising startup out of Idealab, said today that it has acquired Echofon, the developer of a number of Twitter applications. Financial terms of the buy were not disclosed. Echofon develops applications which allow users to read and post Twitter messages, and which run on the iPhone, iPad, and Firefox. PostUp CEO Bill Gross--who also runs Idealab itself--said in a statement that the buy will complement its existing Twitter app, Twidroyd, which runs on Android phones. PostUp offers up a service which allows Twitter users to increase the prominence of their posts in searches on third party websites and Twitter client applications, via bidding on keywords and topics.