Not a comics creator here, nor do I have experience when it comes to subs on Facebook, but...
I'd be wary of them if I were you. The myth that FB owns your photos/pictures is not 100% true, of course, but looking at their terms of service, I fail to see a big difference between actually owning user-generated content and, well, that:
you grant us a non-exclusive, transferable, sub-licensable, royalty-free, worldwide license to use any IP content that you post on or in connection with Facebook (IP License). This IP License ends when you delete your IP content or your account unless your content has been shared with others, and they have not deleted it.
Think about it:
A “royalty-free worldwide license” means Facebook is free to use your photos pretty much how they’d like anywhere in the world without paying you a penny or asking your permission.
It may be "subject to your privacy settings", as the linked article states as well, but assuming that you'd post your work there publicly... well. Facebook would only lose the right to use your work when you delete your account, and then you would have nothing.
It's your decision, but looking at these facts alone, I'd advise against it;;