imagine usernames were centralized on the entire internet 
that would actually suck but once you got it you don’t have to deal with having different usernames on different platforms
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it just makes sense. companies/celebrities who need their clean brands can pay for their .coms, and theres plenty of TLDs for the rest of us.
although it is a bit imperfect. while adding a cost to usernames does make them a little more permanent, the fact that they can expire leads to lots of edge cases that would have to be handled.
that’s true, we should just have phone numbers as usernames at this point
The simpler, more memorable ones would run out over time if the billions of ordinary facebook users all had to have domains…
You’d just have to get to pick your username first. 
If you’re not early, all the good usernames would be gone 