Anecdotally, I spent a couple of months this summer going through team selection at Google NYC, with a specific request for back-end, low-level, preferably C++ roles (app security, build systems, etc.). I didn't meet or talk to a single woman through the entire process, with the exception of reception, another candidate, and someone who was covering for my recruiter when he was on vacation. I asked every manager I talked to about diversity on their team, and they all gave wishy-washy answers (at best; one told me that women just weren't interested in his team's work). If I'm remembering right, of the teams that I could have accepted an offer from, exactly one had exactly one woman.

This wasn't the only reason that I ultimately turned down the Google offer (and spent so long in team selection after passing the interview), but it was certainly one of them.

There are some back-end teams at Google that have lots of women; Chrome security comes to mind (I don't know if that's "back-end" in the common sense of the term, but it was the sort of team I was interested in). But I don't find it particularly dubious that these are the exception and not the rule.