They're just turning the big dial that squeezes more money out of every user. They resisted for so long, but now they can't grow YoY without turning it. It's a one-way dial too, you can never turn it back.

Yahoo did this, most easily seen back on Yahoo Messenger, where I believe they fit an ad in 6 point type between two elements.

I've seen it at plenty of places that does revenue based A/B testing. You do an A/B test, measuring $ vs User retention. Money goes up, and retention maybe is affected, but over a few weeks, it looks like its the same. So you go on to the next test. Eventually, you find you have to do drastic measures to stop bleeding money. I wish I could say how long you need to measure retention for, but its probably too long for the test to make sense.