They do this by exchanging follows. There are people who'll follow you if you follow them back. When such people get together, one's following grows quickly, but that's not a real following.
Actually, a brand-new twitter or facebook account could use some 1000 fake followers just to have weight in the eyes of the real followers it's trying to attract.
But, then, if you're doing social media for traffic and conversions, fake followers do you no good. Then you'd want to manually pick what to post and who to follow. This can be partially automated as well. For instance, I just began using the buzzbundle tool, it helps find relevant tweets, retweet them, follow those people and chances are they'll follow you back.