I understand why, the software works really well. But given the right amount of engineering resources, anyone could duplicate it. Nothing is keeping you on Dropbox other than the quality of the software, so if better quality software comes along, you'll switch to that and Dropbox will be a distant memory of the past.
Compare that to Facebook where you can't switch because your friends won't switch, and you only use Facebook to interact with your friends. Even if there is a better social networking site than Facebook, it won't matter because nobody will use it.
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.
Dropbox has shown the huge need people have for their services. Why wouldn't the big guys (Apple, Google, MS) be compelled to now get in the game and make the investment in treating platforms other than their own as first-class citizens?
One possibility is the amount of money to be gained. According to http://www.quora.com/What-is-Dropboxs-revenue, "revenue is on track to hit $240 million in 2011". Google, on the other hand, made $37,905 million in 2011. Microsoft made $69,943. Apple requires too much effort for me to get the numbers, but it's more money than either Microsoft or Google.
So my theory is, "why bother?" If one of those companies needed a few more million, there are easier ways of making the money.
Compare that to Facebook where you can't switch because your friends won't switch, and you only use Facebook to interact with your friends. Even if there is a better social networking site than Facebook, it won't matter because nobody will use it.