Watermarks have no specific legal merit and never did. People use them because it makes them feel better, copyright exists with or without the watermark and Facebook's terms (that you agreed to) apply with or without it.

Plus implicit agreements to a contract is a rabbit hole, and you'd never be able to show that a human working at Facebook ever witnessed this supposed contractual verbiage.

This reminds me a lot of those "share this image to stop Facebook selling your information to advertisers" meme that was going around a few years ago.