Flashing your hazards to thank somebody isn't something I've even seen officially recommended and I see it more as a nuisance in most cases. The communication can't really be unambiguously understood as a "thanks", especially not if you do it for more than one flash.

It's something I personally do about once in a hundred passes if I think somebody really went out of their way to accommodate me, but even then I realize it's detracting from road safety in several ways, such as reaching over to the often awkwardly placed button, and perhaps confusing other drivers.

But I suppose it's a cultural thing, I've heard that in India it's common to use the horn to notify other drivers of your presence even though there's no imminent danger. Which is something that in most other countries would be seen as obnoxious.