Rules
Letters, numbers, underscores and periods. No period at the end. One change every 30 days.
Why short ones are rare
People registered them early and keep them.
Option
A variation that matches your other platforms.
Official sources
Platforms change their help pages and forms often. Follow the wording on your own screen and the current Help Center. Ogclaim is an independent service and is not affiliated with Meta or any platform named on this site.