Yeah, filing a trademark doesn't cost that much in the grand scheme of things, especially when you've built up as much brand equity around the name as 53 has.

A lawsuit on the other hand would burn through cash, but IANAL so I don't know if the damages they could extract from Facebook would be worth it. Maybe being in a lawsuit against goliath would make the lawsuit pay for itself as free advertising, since I reckon journalists will enjoy writing about such a case.