You can be sure that any system at Yahoo! or anywhere else that is the bread and butter of the operation (ad systems, accounting, fraud tracking) still involves a software QA team.

Facebook could afford to "move fast and break things" when they weren't taking money from anyone. Now that they're a real company, that kind of attitude can get them adverse attention from the SEC if it causes an accounting discrepancy or from trial lawyers if it upsets their advertisers.