I also run an eCommerce website on a relatively small scale. I have found SEO to be the best method for increasing traffic / sales.
I am in a different market to you and although we do have a Facebook page, it does not generate any sales.
It is probably under utilised to be fair.
Twitter has minimal benefits for me also.
I use Google AdWords for the UK market as my .ie site cannot break into the top 10 results. I find it works but it certainly eats into the profits.
Something which does work for me is good old fashioned leg work! We have leaflets placed in all major Irish hospitals and a number of UK ones. My target market is patients with casts / bandages etc.
Talking to nurses, doctors, generally increasing brand awareness brings sales too, it's not all about the web.