Dating is an imprecise word that covers both more casual relationships and the process more rightly called courtship. I am not familiar with any sites that list themselves as courtship sites. (Sites offering that service are typically listed as dating sites, though some dating sites explicitly cater to either/both.) Courtship is the process of sorting out who is a good match for a long term relationship and forming that bond and so forth.

Courtship typically requires one to also sort out how you fit into the larger social fabric of each other's lives. In contrast, hookups or casual flings typically are looking to cut you out of the fabric of their social lives. People generally don't want to introduce casual sexual relationships to important people in their lives, thus expressions like "The kind you don't take home to mother."

Facebook is a social network. Their value position naturally readily aligns with the idea of courtship and they are apparently going with that.

I don't use either dating sites or hookup sites. I'm not the target market. I'm just making observations based on reading up on how asymmetrical markets work and this is a standard thing that you run across. I have read that it is why you see "Ladies night" at bars. The guys are going to be there anyway. You need to find some means to entice the women to show up.

(Disclaimer: I also don't drink, so this is just stuff I have read about working with asymmetrical markets and developing a service when you have a chicken-and-egg problem.)