The coders who join these companies are not in it to do the right things (cf. the popular things), to make sound decisions (even if they are based on "old" thinking, like the notion of privacy, and appear to be boring). These developers are in it for the perceived prestige of working for a high profile company, for the nonserious working environment (if they are young), and often simply for the money. These jobs often pay better than, e.g., working on projects where there is great concern with doing things the right way. But the high-profile "Web 2.0" company does not necessarily produce better code.

Doing The Right Thing (tm) is always easy when looking from the outside in. In my experience, good developers write shitty code all the time, because the world is a messy place and quality comes at a price.

As an anecdote, I once worked on an application where we had to store passwords in plain text, because the password should also be used in a piece of hardware and to provision that hardware, we needed to send the password to said hardware. There's just no good and easy way to solve that and when you're constrained by real world limitations, sometimes you just have to accept a suboptimal solution.