Devil's Advocate; Raspberry Pis don't use traditional advertising, but they advertise a whole lot. Your example of the YouTube video -- while yes the channel may have gone out and purchased the device based on word of mouth because it was cool, there's an equally likely chance that the Ras Pi foundation sent out devices to traditionally "smaller" channels since they want to create that word-of-mouth advertising feeling, despite spending a whole lot of money on advertising.

This could be because they know that their demographic is precisely the type to avoid traditional advertising, be it because they run ad-blockers, or don't consume media with built in advertising like television, etc. Or maybe it's because they know that those smaller channels have significant viewer overlap with their target demographic, so why bother with trying to find these customers yourself when someone else already has them (for lack of a better term) captive?

I've personally seen Ras Pi's mentioned in "niche" magazines a few times, usually when a new generation of the device comes out. That's 100% advertising, although I prefer it to a generic "buy our thing" ad, because at least they go into detail showing me what exactly it can do for me.