I think it is growing, I've been a triple screen gamer since the nvidia geforce 4600ti+440, it was not true SLS tech like eyefinity, but it was possible to stretch a windowed game across this space, and have it working, not fast admittedly, but back then games where also less demanding.
We are seeing more and more games now support multi screen, and many of our older favs have had workarounds found by various members here too.
We also see a lot of new members joining our forums and enquiring about what adaptors to use, what gpu's to purchase and how to get various games working.
We also have quite a strong multi-monitor user base
here.
Companies marketing will try push for 4k displays as the next big thing for the average consumer, try pry them away from 1080p, while those of use with triple displays are already using 3k.
Fact is that it is now becoming much more affordable than was previously and a hell of a lot less complicated too now that a number of cards require no adaptors at all to run 3 displays.
Friends to whom I have shown my rig all love it, and al wish to recreate such a thing, but where previously limited by cost, they now see this as a viable and very immersive way to go.