Question: I am looking for a single word to describe a group of customers, who merely take advantage of the company's services (they come only for free period and leave afterwards or take the gift and leave again).

So far I came up with a term `Profiteers`, but this is somewhat pejorative, which is not what I want.

Answer: 
In the specific context of *credit cards*, the word “tart” (in the sense of prostitute) is used in combination with **rate** or **card** (at least in Britain). Thus, on [macmillandictionary.com](http://www.macmillandictionary.com/buzzword/entries/rate-tart.html):

> 
> **rate tart**
> 
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> noun [countable]
> 
> 
> 1. *also* **card tart** or **card surfer**   
> someone who continually switches credit card providers in order to get the lowest interest rates
> available
> 
> 
> 

Sorry, it’s not a single word, but the combinations represent specific usage. (And it’s not plural either, but you can add an ‘s’ if you like.)
I like the term fleeting for this usage.

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> [Fleeting](http://www.dictionary.com/browse/fleeting) - passing swiftly; vanishing quickly; transient; transitory:
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> 
> 

It know it doesn't fit perfectly but it closely describes people who drifted away from my product after the free trial period.

Or you could use Itinerant.

> 
> [Itinerant](http://www.dictionary.com/browse/itinerant) - a person who travels from place to place, especially for duty or business.
> 
> 
> 

This could mean that the person was only using your product only for completing an item before moving on with the process (to possibly someone else's product).