One of the things I teach tech writers: it’s okay to have fun with your audience most of the time. But never joke with frustrated people. If you’re delivering bad news, an alert, or helping a new user troubleshoot a problem, be straightforward and transparent.
As I’m sure Amber would be the first to say, said rule is not just for tech writers. In any online interaction, synchronous or asynchronous, with someone you don’t know really, really well, that rule applies. Always. And the defense “Hey, man, where’s your sense of humor?” is usually the reddest of flags that you have already screwed this up, and should consider apologizing.
(I’m especially looking at you, Twitterers.)