Some people are pointing out how rare it is for GitHubbers to express gratitude, but GitHub doesn't exactly encourage it. Doesn't it feel inappropriate to express gratitude as an "issue"? Maybe if there was a natural place for these messages they'd be sent more often. To the extent that there are affordances for it (stars, emojis) it is actually quite popular to praise good work on GitHub.
I do, then, immediately close "thank you" issues so owners are not tasked with responding in any way, but still... And for any watcher, I imagine they are interested in notifications relevant to actual issues, not thank yous.
My opinion is the current best way to say thank you is probably a star. Beyond that, it would be nice if GitHub added a way to leave a thank you or testimonial or something like that. Or even a formal "the repo is in use by such and such app/company."