Most small businesses fail at this because...

They don't have the time

Most small business owners don't have enough hours in the day and even though they start with the aim to post regularly, this soon stops when they're pushed for time.

They don't post often enough

Just one post per week, or even less frequently is just not enough to been seen online and get real quantifiable results

They don't post where their customers are

As you've seen, Facebook is the most popular social network, but lots of your customers are on Twitter, Pinterest and Instagram too.

They don't post the right content.

Your posts need to be the right mix of promotions, info about your products and services and posts that encourage engagement.

Engagement on posts (ie comments, likes and shares) is important because Facebook then sees the post as popular and so shows that, and your other posts, to more and more people. Posts should also use quality images that catch the reader's eye in the newsfeed.