To be honest with you - and this is not intended as criticism in any way - but if I had to guess, I'd say it has more to do with the type of item you sell than anything else. Video games don't strike me as the kind of item where buyers are looking for / needing a personalised service, and while you have a lot of eBay members who will leave FB for every transaction no matter what it is they buy, I think there are some categories where buyers are less mindful of - or place less value on - leaving FB for a selller, particularly positives.

If you think about it in terms of say visiting a normal shop where they have feedback slips you can fill out - what kind of things motivate people to actually go to the effort of doing so when it's much simpler, and faster, to get what you want and leave... (The two things being exceptional customer service and poor customer service, with - I'm guessing - fewer people filling one out for great customer service; they're more likely to just tell a few people about it or post on social networks, i.e. mediums that are still grabbing attention because they're interactive and they'll get a response, which is more gratifying that eBay's static FB system IYKWIM).

I don't mean to suggest you're not offering service good enough to deserve feedback, I'm just trying to point out the (possible) mindset behind people not even thinking it's worth coming back to post a FB comment. 

For a comparison, on my main ID I sell handmade items, quite a few are custom made, so there's often a little interaction between the buyer and myself before and after purchase; even if not, I send every buyer a message with some key information they don't get from eBay's automated messages (this is just the way I like to do things, and I know it's not always possible or appropriate for other sellers), and even then I'm at about 80% FB returned (that's calculated over a few years, and I know for certain the percentage would be lower if based on the last 12 or 6 months - it does seem buyers are increasingly disinterested in leaving FB no matter what you sell). 

This is all just opinion, anyway.