Twitter Following

Hello,

I've noticed that a lot of twitter accounts are starting to follow lot's of people and in return their following & retweet numbers raise. I was wondering a few things, are these accounts following certain groups of people and if so who? Also how many people can I follow at a time?
I have 201k REAL followers and just started following about 3.1k in the past 3-4 days.

Thank you everyone who replies to this!

Anyone?

I don't know about the followback lists but when I started up with twitter I followed a lot of people on others followback lists. Then I followed everyone back that followed me and people would list me on their list. I know that twitter cuts you off when you follow back too many people at once. I think they start limiting the amount of people you can follow per day after you are following 2000 people. traffup is a good site for getting followbacks and follower exchange.