And then I checked out Alexa's top list of most popular English language websites. Believe it or not, in the top 100 sites seven or more (depending on how wide you set your criteria) are simple file sharing/hosting/sending services.
Of course, Alexa is known to be very imprecise at times, but in this case it doesn't matter; it's obvious all these sites - Easy-Share, Filefactory, 4shared, Megaupload, , MediaFire, Zshare, and the biggest one, sitting at the 42nd spot of all English-language websites, YourFileHost - are all serving many millions of users daily.
Interestingly, some of the more recognizable names, like Microsoft's Live SkyDrive, or the aforementioned Box.net are in the top 7.
And I'm not even counting image hosting services, like Photobucket, some of which are even bigger.
If you had asked me yesterday, I'd estimate that perhaps one or two file hosting services are ranking really good, with thousands of others being significantly smaller. However, it turns out that there's a spot for everyone in this business; I can't remember any other web service, besides social networking, with so many high-profile competitors doing so well. Upcomers include torrents (the Pirate Bay recently entered Alexa's top 100 list, while Mininova sits at 52 worldwide) and porn; but if you look only at the top 100, neither is as big as file sharing&hosting.
All of which brings us to an interesting question: what the hell is Google waiting for? Rumors of Gdrive (or GoogleDrive) are at least two years old, and they pop up every now and then, but Google is still sitting on this idea. Why, since it's obviously a very lucrative business? Do they have some groundbreaking technology up their sleeve, and they're just waiting to release it and kill the competition in one clean sweep? Even Microsoft has answered the call with their Live SkyDrive, and I'm really anxious to see what the big G plans for this space.