I once used the built-in contact form in a webapp to tell the creators that one of the ads they were showing was malware. They thanked me, and I got a couple of

I once used the built-in contact form in a webapp to tell the creators that one of the ads they were showing was malware. They thanked me, and I got a couple of free months' premium service out of it. I was very happy with that - it was a relatively minor complaint on my part, I was a free user, and they got a paid customer for a few months after their gift expired out of that. Generally I think that'd work for the majority of free users of a service. For paying customers, though, you sort of have to triage between minor complaints and ones that will lose you a customer quickly if you don't look like you're solving it.