Post, engage, involve, no secret. It doesn't matter whether it's Facebook, Twitter or any other platform, the rules are much the same.
If there is any secret it is the art of actually doing it. Every day you should set a little time aside to work your social media. On Twitter if you follow people there is a chance they will follow you back especially if they see there is a chance you will retweet some of their posts. As you get a larger following you will get increasingly more followers. You'll also attract a lot of followers trying to sell you stuff in a less than subtle way hoping you will retweet to your followers. Retweeting posts that maybe related to your business activity or ethos is never a bad thing.
The same principles apply to Facebook and LinkedIn. Don't just post stuff promoting your product or services, the chances are the only businesses that will notice are the ones wondering whether they can sell to you. Get involved, retweet, share, repost items that interest you or relate to what you do. Comment on others' posts and get a conversation going. I actually won business from a company after madly disagreeing with them on twitter.
Just doing it, that is the secret.