I got to 500 followers on Twitter fairly honestly. Some follow everyone under the sun in the hopes they'll reciprocate. In fact, there are even lists of people on Twitter who automatically follow back everyone, so if you follow 500 of those guys you'll hit 500 followers pretty quick. I only follow people on Twitter I actually want to follow, and right now that's 134 people.
I hit 500 followers at some point last week. I only really got into Twitter this past February, so most of my 500+ followers came on board since then. Unfortunately, there's a dirty factoid about one's Twitter follower count that tarnishes the whole thing.
A good number of Twitter followers are pr / marketing / spammy accounts. Someone is selling something or some service, and they create a Twitter account to market their product and/or service. Then, they follow everyone under the sun, in the hopes you'll reciprocate. I never reciprocate, but I don't do anything to block them from following me, either. So my Twitter follower number appears to exceed 500, but the number of actual people reading my tweets is most definitely less than that.
It's the dirty truth about Twitter followers. And the more Twitter followers you have, the more pr / marketing / spammy accounts you'll have following you. The cosmic Twitter ballet, continues...
P.S. Follow me on Twitter. If you have an account, follow me at http://twitter.com/torontomike.