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.

Microsoft isn't agile enough, Google thinks you should just use web apps for everything, and Apple's already going a different direction with iCloud.

I don't understand why, either. Any good articles on what makes Dropbox Dropbox?

There are articles on the subject, I can't recall them off the top of my head. The gist of it is that Dropbox is more than just cloud storage. They've put an unbelievable amount of work into the syncing client itself on all platforms so it's really robust, very cross platform, while at the same time uniquely tailored to each platform and completely seamless in operation.

Here is a little snippet from Quora on the subject, sorry I couldn't find anything more: http://www.quora.com/How-does-Dropbox-integrate-with-OS-X-fi...

And anyone can clone it, which is the point. If Google decides to launch Gdrive, it would dwarf Dropbox instantly. Whereas even if you were given all of Facebook's hardware and software and staff, but not user accounts, you would have a hard time competing with Facebook.

I'm not sure about that. Google Music's sync client is horrendous, as were Google Desktop and Gmail Notifier. I wouldn't trust Google to be able to make a client on par with that of Dropbox. (Remember, Dropbox engineers actually reverse engineered the Finder and performed code injection in early versions to get the user experience they wanted.)

A singular devotion to UX is not something that can be duplicated given "the right amount of engineering resources". (jrockway's thinking is totally emblematic of Google and Microsoft.) It requires the right, most product-focused people in charge.