Mine looks pretty similar:
^.*\.?(facebook|.*fb.*?)\..+$
Your opening match clause ^(.+\.)?? seems to be more efficient than both my approach ^.*\.? and the one Pi-hole inserts for wildcard entries (^|\.) - more efficient in the sense that regex101.com shows some 8 or 38 steps less needed to evaluate yours (using other-sub.graph.fb.com as test string).
Of course, that's no hard assessment criterion, as Pi-hole's runtime behaviour might differ, depending on the actually regex implementation used.
Midways, mine would still match fbcdn or tfbnw parts, as I apply the leading and trailing wildcard matches in the domain part that you cut away from yours.
Towards EOL, mine would also catch country specific TLDs like .co.uk or .nl, but may overblock, e.g. by also matching facebook.someblacklistingdomain.com.
I am going to adopt your opening match to my regex. 
And just out of curiosity, maybe a developer could comment whether it actually would be beneficial to replace (^|\.) by ^(.+\.)?? in simple wildcard matching - absolutely no priority though