Interestingly not a huge exit its "just" 3x. (Edited from 6x I looked at investment not valuation originally) For all the news around Jet taking on Amazon I'm not sure this is signaling a win for them.
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[0] https://www.crunchbase.com/funding-round/79d35bb376dcc222a8a...
[1] Math: $500M invested at $1B results in those investors owning 1/3 of the company. Assuming pro rata distribution of the acquisition proceeds, they would get $1B in a $3B acquisition.
[2] http://www.bloomberg.com/news/articles/2015-03-17/the-fuzzy-...
Decent article on this from Fred Wilson http://avc.com/2009/03/what-is-a-good-venture-return/
This is a company which was all but guaranteed to succeed and sell in <3 years for billions.