Your username is your link
On Facebook the username matters most when you share your profile or Page outside Facebook: in an email signature, on a flyer, in another app's bio. A clean facebook.com/yourname is easier to remember than the long address Facebook gives you by default.
Because periods are the only separator Facebook allows, ideas here use periods or no separator at all ("maya.bakes", "mayabakes"). Underscores and hyphens that work on other platforms would be refused.
Matching your other accounts
If you are @mayabakes on Instagram, try to be facebook.com/mayabakes too. Facebook's minimum of 5 characters rules out very short names that might work elsewhere, so a short brand may need a word added ("mayabakes" rather than "maya").
When your first choice is taken, add something meaningful, such as your town or what you do, rather than a string of digits. A link with a city in it still looks like a real person or business.