There are lots of services these days which for an amount of money (let's say $2 per fan) will work on your behalf to gain fans and followers on popular social networks such as Facebook. Now, there's two ways these services go about doing that.
Unincentivized
The services are just buying ads on different sites (more likely on Facebook itself) where they advertise either your Facebook page or group. In my opinion, that's just advertising and is ok. It serves the purpose of informing people about your group/page that are interested but simply didn't know it existed. Since I am using a sex analogy, this to me is the equivalent of paying for a profile on some adult personals site through which you find a "lover" who otherwise wouldn't have known about you.
Incentivized
In the case of incentivized fans, these service providers offer users something in return for joining your page or group. Now, if that something would be an incentive relevant to your business (a free guide, downloadable content, etc), then this would almost be an ok strategy. However, that's not how it works. It's more along the lines of "when you join Coca Cola's Facebook group, we'll upgrade your Farmville Tractor." In a way, that's ok too. As a brand, you just need to be aware of what you're getting. You're getting people who're passionate or even care about your products or services but rather a bunch of Farmville junkies who couldn't resist the temptation of upping the size of their next crop.
Going back to my sex analogy, this is the equivalent of paying someone to have sex with you. While that might be ok to some people, just recognize what you're getting. Or better said, what you're not getting. The person likely has no affinity towards you, won't tell his/her friends about you and nor will she/he want to listen to you once the transaction has passed. In terms of your Facebook friends, that means your incentivized buddy really doesn't care about you, won't recommend your products to his/her friends and more likely than not will opt out at the first opportunity that arises. So again, why would you you want to pay for that person?
By the way, paying for sex is illegal in large parts of this country. Joining our Facebook page or following us on Twitter however is not. Today, and today only...if you follow us on Twitter or Facebook, I promise to give you a hug the next time I see you in person. No cuddles afterwards though, just a hug. And please don't expect me to tell my friends about it.