Facebook is a registered trademark, which earns it the right to use the ® symbol.

The symbol capital R within a circle [®], or circled capital letter R, is the registered trademark symbol. This indicates that the product is registered with a national trademark office (ex: Registered, U.S. Patent and Trademark Office). If a trademark is not officially registered, then it will be marked with a superscript TM (in capitals).

Symbol for "registered trademark"

The registered trademark symbol, an R in a circle, indicates the preceding logo, slogan, etc. has been formally registered; the trademark symbol, a superscript TM, indicates the mark has not been formally registered.

It's not required to use the trademark or registered trademark symbols, and some designers feel it clutters the look of the packaging.