Tweetmeme isn't a great example. They didn't get steamrolled—they now power the official Tweet Button, and they'll power (they are?) DataSift, which is going to give people crazy querying capability against real-time twitter data.
Found the link: http://techcrunch.com/2010/08/12/twitter-tweet-button/
I do see mentioning of an agreement with Twitter but it is my understanding that Twitter has a licensing agreement to use the tweetmeme technology not that tweetmeme will be powering everything.
Edit: You could be right about the Tweet Button. DataSift will be run on TweetMeme's servers drawing data from the firehose.