These are two different kinds of moats. Yes, Facebook has very strong network effects that create an advantage. However, it's a mistake to underestimate the strength of Dropbox's engineering complexity and strong product design. It wouldn't be simple for anyone - including Google or Microsoft - to just clone what Dropbox has done. Apple, Google, or Microsoft would have to decide that they were comfortable treating platforms other than their own as first-class citizens, and be able to defend the incredible emphasis on simplicity all the way through the product's lifecycle. Facebook would have to dedicate themselves to QA and quality-rather-than-velocity in a way they've shown they don't like doing. Anyone else, well, like I said: good luck.