So what is the definition of "most successful startup" being used here? Is it historical? Is it just number of users, or acquisition price? It certainly can't b

So what is the definition of "most successful startup" being used here? Is it historical? Is it just number of users, or acquisition price? It certainly can't be revenue if Tumblr is the king. I know it's a small aside in the article but it always irks me how these type of labels get tossed around without any actual consideration by the tosser as to what it means.