Facebook and Twitter have changed their Terms of Services/APIs multiple times which has resulted in bad times for Facebook applications like Vintage Camera and MessageMe[1] and Twitter services like Flattr[2].
However, Tapbots has been successful in adapting its Tweetbot client to comply with Twitter's API changes[3]. While the API changes cost Tapbots extra development time to bring Tweetbot up to snuff, their core business was not adversely affected. I don't think Tapbots has a bad time because they make Twitter clients that rely on third-party API access, but they are at the mercy of Twitter continuing to allow third-party API access.
[1] http://online.wsj.com/article/SB1000142412788732439280457836...
[2] http://thenextweb.com/insider/2013/04/16/flattr-twitter-paym...