And then watch out if your company bases its diversity targets on numbers from the rest of the industry. All the good ones may be taken (in order to get to the mythical 50%), but now we want to follow suit (to get to 50%), but what's left? What a mess.

This is another of the "facts" that Damore's memo mentions but there's no empirical proof for. Do you have any?

Proof for what? Smaller number of available women? That's obvious. That if larger companies hire the best of them, the remaining supply/quality will shrink? That's obvious too.

No, that there's a hard number that Google recruiters are supposed to achieve (50% seems to what I've seen repeated around here.)

Does anyone have any actual proof that Google is aiming for 50% or to represent the general population 1-1?

It sounds like they kept meetings deliberately off-the-record, but then again here on https://www.google.com/diversity/index.html there's stuff like "we’re committed to making our workforce more reflective of the world we live in". Number goals are unlikely to be published for obvious liability reasons.