No, it's not posting too much sales related content.
No, it's not even incorrectly setting your business up as an individual profile versus a public one (Especially now that you can change that).
It's actually this:
Not taking the steps to ensure that your current customers have "liked" you on Facebook.
Think about it. Facebook is all about the concept of Word of Mouth. In real life, if your friend uses something that works for them, and recommends it to you, you are far more likely to use that product or service than if a stranger or an ad recommended it to you. That's supported by data and by sheer consumer common sense. Facebook has just taken that simple concept and brought it to a faster, better place. And they've worked hard to build that network to scale-- so that just about all of your friends are there.
By not getting your current customers on Facebook, you are essentially required to work 10x harder to get likes from people who don't know you and don't yet work with you. When they do finally come to your page, if your customers aren't on there, they'll see a page that features no feedback, recommendations, or insight into how your business is run. They'll just see you talking.
What do you use currently to market your business? Consider going above and beyond just slapping a Facebook logo on there. Consider personally asking your customers to join. Consider a dedicated email. Consider a customers-only contest. There are tons of ways to do it, it's just that so many of us are focused on acquiring new customers through new channels, we forget that the key way to do that is by working with the folks who already know and love us.
Get your customers onto your Fan Page. It will help a ton.
What do you think is the #1 mistake made by small businesses on Facebook? Share your thoughts below!