Dropbox's security is twitter's downtime. While much more is at stake than not being able to tweet, I can't imagine that this isn't their number one growth challenge -- something that, if they conquer it, will give them a much higher market valuation.
If this happens, let's look forward to a trove of blogposts about "how to make dropbox secure" from armchair CTOs, just like we saw with Twitter and the string of posts around "How I'd scale twitter" Sharding! Webscale!
I don't think this is a reasonable comparison. Twitter was at least 80% as useful when its uptime was erratic as it is now, when it's uptime is reasonably good. But Dropbox security flaws potentially cough up your data to criminals; when Dropbox security fails, its utility is negative, not slightly diminished.
If this happens, let's look forward to a trove of blogposts about "how to make dropbox secure" from armchair CTOs, just like we saw with Twitter and the string of posts around "How I'd scale twitter" Sharding! Webscale!