The customers were generally people trying to get their small business out there or some local thinking it could make them into an internet celebrity (I work in/around Scottsdale, AZ and there are a lot of people here that think like that).
We had a separate script that would check to make sure each account was still active once a day and if accounts were removed, they would be replenished automatically, although once a client lost a follower, it was gone and a new one didn't replace it for them.
It was all kinda shady and it was one of the numerous reasons I left that web agency (which went bankrupt a year later and is no more), but it apparently produced decent results since search ranking increased both on Twitter/Facebook and on Google. I'm unsure if it is still as effective, though; this was almost 4 years ago that I left.
Only time I've seen it first hand since was with a client trying to get famous. They made some YouTube videos and bought tens of thousands of YouTube subscribers from somewhere (we don't offer that at my current agency)
That said, the latter example on YouTube was not effective at all, but that was probably because the content wasn't great.
It doesn't matter how 'pure' a site is, you can still buy a link, it just costs more.
It's all about looking at how the algorithm works, then gaming it. Tinker with Twitter and facebook, spend some money, keep the results secret. Sell this information. Do what works to your clients.
Bot nets work. For now. Does Twitter want to kill them? Maybe not, active accounts is a big number that they love to tell people.
If you enjoy chiming in on national conversations then it's a cheap way to appear prestigious. Of course, you need to be able to argue well enough to not out yourself as some random joe.
A look at the history or content of mentions is probably the best bet to detect a faked account. Still, a clever person might slip by a cursory check.
You could totally use it for business purposes too - as a really shitty form of advertising or just to make your popularity look higher. I doubt it'd be too effective, but hey, it's cheap.
I know from my own experience when doing research on a company - whether it's a prospective client, vendor or competitor, I'll click through to their social properties to gauge their traction.
I definitely look at their engagement ratios to determine whether their social following is organic or fake, but I think typical consumers miss this.
I'm actually surprised this is news to anyone. Some of my first Twitter followers were blatantly fake (unless vacuous Russian models are really into JavaScript programming all of a sudden) accounts trying to appear legitimate at a glance -- following random people and posting credibly arbitrary nonsense to avoid easy detection when they later follow/like whatever someone paid their owner to.