I don't think it will spread too much.

Nothing would kill Facebook quicker than then implementing this, for example.

Agreed. I'm on the verge of leaving facebook, I've been for quite some time. Their stupid change in chat (people know when you read what they wrote) almost made me quit, but Chrome extensions to disable it allowed me to stay.

With such a move, I would not stay one day longer on the "service".

It's boiled frogs all the way down!