Facebook and Twitter, along with other digital media sites have one thing in common. They are all just tools.
They are networks of real-live people.
You seek out the ones who already know you, who are already your disciples, who are like-minded, have the same interests, and perhaps friends of those individuals. You would treat them the same as you would in a face-to-face environment: with respect.
+David Meerman Scott describes social media brilliantly in his book The New Rules of Marketing and PR. He likens social media to a cocktail party. You wouldn't go to a real event and start (spamming) feature-dumping and "buy my stuff" your business to anyone who will listen, even if you just met. Or would you? Or would you be someone hiding in the corner with your face to the wall, hoping no one will talk to you? That reflects the people who set up profiles and then never engage or visit their networks.
You will have the best time at the cocktail party when you take the time to say hello (connect), get to know other guests (see what others have posted, like and make a positive comment on their feed), and share good stories (make your own posts interesting and ones that will elicit engagement).
When it comes to working the networks to promote your brand, think like a content creator, not like a content consumer.