We have a Facebook page and we tweet regularly on Twitter. I do think it helps you build your readership, and it definitely helps bring customers into our store.
I should have given you a specific example of what it's done for us this week. A giveaway our store is having on Facebook has already gotten me 65 new blog followers in the last 4 days, and the winner won't be chosen till Sunday. Admittedly, this experience is far from ordinary, and the giveaway has taken on a life of its own...to the extent that we had to ask people to limit their sharing to once a day. With nearly 1200 shares already, it was starting to seem "spammy," which was never our intent. (Our Facebook page has picked up 827 likes in the last 4 days...just totally nuts!) But regular, coordinated use of social media has been mutually beneficial to my blog and our store, even before this giveaway craziness.