The answer is yes and no. Social media sites do indeed not pay you for your content, but their platforms (in most cases) is not really for providing stories or articles, but rather the 'social' component. As such, the negative effects of contributing to a non-paying market are reduced.
Furthermore, what you write has a huge impact: a novelist providing a piece of flash fiction for use on facebook, or making a short twitter story is very different from providing a full-fledged short story.
However, there is a case where it definitely isn't the normal meaning of 'writing for exposure': writing a blurb on facebook, interacting with fans on twitter. While you are writing, and doing this for exposure, you are clearly not providing your product for free.
So to sum up:
a) if you are providing a form of your product on these platforms, you are writing for exposure, but within a different 'marketplace' from your usual one; which means that it's not as bad as giving your stuff away to a magazine for free.
b) modified, smaller versions of your product (as in a fiction writer providing a hundred-word musing etc.)that are easy to produce : this is writing for exposure, but can often be quite easy to justify when the costs (time spent) and loss (not being able to sell that work) are taken into account.
c) If it isn't your product- i.e. it's promotional work, teasers, musings etc- it is called marketing. The only question is how effective it will be, and whether it's a waste of time or not.