Hi Fairy Cardmaker,
There are many benefits to having FB & Tw accounts, other than just reaching your direct customers.
FB and Twitter annoy the heck out of me and do not represent my target market anyway.
From SEO perspective, those sites are adding backlinks to your blog and your store. Backlinks matter in turms of raising your online rankings and getting more visibility on Google. Backlinks have to be diverse and would preferably come from various social media sites too.
Google changed their algorithm in the last year or so to include social signals as part of evaluation your pages importance.
Another benefit from having links published on FB & especially Twitter is that Google almost immediately adds Tw updates to their index (and displays your pages in their replacement results pages). This means your new or recently updated pages will be crawled (found) by Google a lot faster, because Google robots will follow that link from FB or Tw, especially if your posts on those sites get a lot of Likes and Retweets.
Plus, there has to be someone interested in your products out of combined 1.5 billion people that use Twitter and FB.
Gosh, I don't have time to keep up with these things.
Lisa, if you don't have time to maintain FB & Tw accounts, I'd automate your updates to them, but I would still have those accounts for all those reasons.