A principle that applies in volunteer communities, online, and in the workplace is: be specific. Saying "thanks" is nice -- and if you can't do anything more, at least do that -- but as you've noted, it doesn't seem to have as much impact as you were hoping. That's because "thanks", without any other elaboration, can feel formulaic. It's easy; you don't really have to think about it. Some people will hear it as pro-forma, not heartfelt.
On the other hand, I have seen both volunteers and employees delight in specific praise. Things like:
- "You did a great job de-escalating that tense chat exchange."
- "Thank you for dedicating so much time here. In this last month you've handled over a thousand flags!"
- "We got a really nice letter from such-and-such client praising your work on our newsletter."
- So-and-so (junior team member) looks up to you as a mentor."
I'm a moderator on Stack Exchange and have been a leader in my congregation in the past (board member, committee chair, that kind of thing). The thanks that have meant the most to me were the ones that came with enough elaboration to show that the person saying "thank you" actually had some clue what I did for the organization. I've seen that reaction in fellow volunteers both online and in physical communities.