Also, tragically the world is wired to provide feedback mostly of the negative kind. This is useful to receive but also it's an unfortunate skew. Positive feedback with a few details about what is good are a hugely valuable contribution.
But if you don't have that amount of time, saying "thanks" alone is worth the keystrokes. :-)
I disagree with that culture. I'd prefer we all exchanged the small "thank yous" even in github threads, code review, etc... but knowing the majority seem to feel it's spam I find most of the time I feel pressure not to write them.
Maybe a few "leaders" like Linus or whoever came out with "say thank you" would help?