But I just wouldn't do it under any circumstances. Not even once.
It's the genuine engagement that counts
The whole point of social networking is to make meaningful contact with real people. You slowly build up trust and rapport by communicating with them repeatedly, however briefly. Much like search engine optimization, it's best to see social media activity as a long term strategy. It has benefits because it isn't easily achieved.
Why have a bunch of followers that you haven't engaged with, and probably never will? This would be pointless even if they were living, breathing humans. And if you buy them in bulk, odds are that many won't be.
A follower "boost" will wear off
Certainly, there are some people who would agree with the above observation, but would consider a one off purchase as a bit of a "leg up" -- a kick-start to their social media careers. I can see the logic in that. And yes, if your profile says you have thousands of followers this will surely impress a certain percentage of real people and thereby make them more likely to follow you.
But if you don't subsequently engage meaningfully with those followers who joined as a result of your purchased boost they won't hang around for long. One by one they'll unfollow you and you'll be left with a bunch of bots or people who just aren't interested.
Much better to have a hundred real followers who genuinely see you as an authority -- many of whom you engage with regularly -- than ten thousand who don't give a toss about you!