Something like "Social coding startup GitHub, hugely popular in the coding world, ...." would be a whole lot clearer.
Little-known, though technically correct, implies that they are not well known in any circle.
WSJ is supposed to provide good reporting with strong bias towards informing potential investors (as a financial/business news paper). The wording choice doesn't make a strong case for value associated with the company and sounds like some no-name co. just got a pile of money that they are like to lose. (An exaggeration on my part for illustration purposes only.)