This is false in Dropbox's case:
While on a GAAP basis, Dropbox is losing money, the company generated $0.18 of non-GAAP earnings per share in 2017, an incredible feat for a software business of their size and growth.
Their GAAP operating margin was (10)% in 2017, but the non-GAAP margin was 5%. Their free cash flow margin has been improving dramatically over the past few years, and was at 28% in 2017, up from 16% in 2016 and (11)% in 2015. [1]
I wouldn't say the same thing about Uber, because they seem to have a negative operating margin, unlike Dropbox.
This is the whole point. Anyone can look at the final result and sneer "unprofitable", but it's meaningless without actually looking at the figures and understanding what their cashflows are.
[1] https://medium.com/@alexfclayton/dropbox-ipo-s-1-breakdown-3...