Not a startup, but in terms of market share, number or regular users etc. it's comparable.
+1
For the curious, I published a logistic model of bitcoin adoption by speculators
https://docs.google.com/spreadsheet/ccc?key=0ArD8rjI3DD1WdGhDN3FBWFptTlZTREN0cFkxZ3JHTnc. The model does not predict the maximum bitcoin price, rather it says when exponential price growth ends for your guess of maximum price. I plotted four plausible maximum per bitcoin prices in USD: 40,000 , 100,000 , 1,000,000 and 5,000,000 . The curves show that exponential growth can continue for a few more years.