Beside if it was a really bad idea, why would both google & facebook guys implement it?

It's not a really bad idea in general. It's not a helpful idea for certain sites. You can't blindly apply performance tuning from one site to another and expect it to work. You have to profile what's slow and what's not to find out if you can expect any improvements or if you're only making it worse.

You haven't profiled anything. Other people have. Listen to them.