This answer is informed in part by your comment that these numbers are not increments of a thousand... And in part by Don't Make Me Think.
My first thought is "are they good for UX?" probably not; truncated numbers may take up less space, and they can be integrated well into a layout, but there is an increased cognitive load associated with determining their value. Small but ultimately greater than plain old numbers.
My next thought is: be consistent, I would suggest against changing base units within the same context (say if multiple follower/subscriber values are listed in one view). The cognitive load is compounded when the values aren't consistent.
Compare the first and second columns below. I don't think that I have to argue the point that the second is not only easier to read and to determine the relative values between rows but it also looks a hell of a lot better.
I think it's even more clear if they're in rows and not columns:
7 4K 133 889 1.7K 2K 21K
7 4,000 133 889 1,700 2,000 21,000
It obviously all depends on your formatting and the context of use. If "followers" only shows up once, say on a profile page, and it's one of few numbers on the page, and even then unrelated to the other numbers, then consistency is less important.
If the numbers don't appear next to each other, such as you have 10 "cards" on the page for profiles/services/feeds and the followers values is listed below the title of each card. The follower values aren't all sitting right next to one another to be compared and at a glace it might even be more obvious that the profiles with the "K" in the number are the more popular ones.
Even if the numbers are next each other but don't directly relate to one another then you can get away with mixing things up and have it still work really well. Take this from an SE user profile for example:
But even then it would work, and possibly better illustrate the actual volume, if it was fully written out as 113,000.
Now 113K may be more obvious than 113,487. It also may be true that the number doesn't need to be exact, that an exact measure isn't important in your application, and then a rounded number might actually be simpler because the user is only expected to casually glace at it and really not understand much from it. Then recognizing a couple of digits and a "K" is going to be really fast.
Use your own judgement but at the end of the day just realize that they are probably going to be a little harder think with and compare, particularly if you're changing base values from 1s to Ks within a context.