Presumably they didn't want to spend a tremendous amount of money for no clear purpose?
Most outfits which registered a brand or company name as a TLD are purely throwing away money here, either because they didn't understand what they were doing or out of sheer vanity.
You can maybe make an argument for a handful of very big technology companies that have some sort of plan for what they'll do with a TLD, such as Google, but I don't think Facebook would be on that list.
