We had a wonderful, thriving life science community [1] on Friendfeed, which we have not been able to replicate since. Some of that crowd even published a paper about it [2]. Most of the original crowd left sometime after the FB acquisition. While many of us hang out on Twitter, it's not the same thing).
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1. http://friendfeed.com/the-life-scientists
2. http://journals.plos.org/ploscompbiol/article?id=10.1371/jou...