I'm somewhat surprised that a file sharing company can get a valuation this large. Don't get me wrong, the product is great, but I'm skeptical that the "file sharing industry" is big enough for valuations like this. Some quick math: A 5 billion valuation implies earnings of at least 250 million/year. Assuming paying customers are worth 240/year and represent 5% of the user base, dropbox would need roughly 21 million…
I believe that everybody will have a least one "cloud storage" account. Actually probably multiple "cloud storage" accounts since cloud storage providers will try to specialize and differentiate (i.e., you will have Google Docs account, Dropbox account, iCloud account, etc.). So 21M is a conservative number. And don't forget companies of all sizes are going into cloud: so replace 21 million users with 21 millin users…
In short, I think Dropbox got on to something big early on, but they will eventually be like the early Netscape of this area.... Other competitors are pretty much guaranteed to come in and erode their value with "free" (enough) alternatives that exist to prop up other business models.