“what does PoW require here?”

Are you asking about how to technically implement it? 

Devs would probably need to make protocol changes to attach a hash “proof” with each message.

Each proof would have a “difficulty” meaning the computational effort required. Presumably this would be dynamically adjusted over time. 

Your phone or computer would do this in the background. All you would notice is the delay, the battery consumption, and your phone getting hot. 

Take down meta? 🤔🤷‍♂️ Does it need to? Personally, I’m happy chatting with a few freedom loving bitcoin maxis. 😊