Rules
3 to 15 characters; letters, numbers, at most one hyphen, underscore or period; start with a letter, end with a letter or number.
Change limit
Once every 12 months.
Tip
Pick it last, after you have checked the other two platforms.
Official sources
Platforms change their help pages and forms often. Follow the wording on your own screen and the current Help Center. Usernm is an independent service and is not affiliated with Meta or any platform named on this site.