I think it comes down to funding, marketing, development and revenue models. Many companies succeed in funding, then effectively jump the shark in terms of marketing spend and try to massively scale their developer teams in order to increase feature churn, which has the opposite effect a lot of times, and then they've burned through all the cash before they can handle the scaling.

Other companies don't go full board, with more conservative growth in terms of features, but can scale more customers without the draw of huge marketing or in some cases smarter marketing. Viral-peer marketing works as long as your conversion out-paces your free sign-ups. I don't have any insights into Dropbox, but can say they're probably fine. There really isn't great competition in this space.. there's competition, but none of the cross-platform options are as good or at least not clearly better.