I'm going to agree with the people saying that agreement often has little to no useful information content (the irony is acknowledged). Note, for instance, that content-free "Me too!" posts have been socially contraindicated on the internet since time immemorial, and content-free disagreement is also generally verboten. This also explains the conference example, I expect. Significantly, if this is actually the root of the issue, we don't want to fight it. Informational content is a good thing. However, we may need to find ways to counteract the negative effects.

Personally, having been somewhat aware of this phenomenon, when I've agreed with what someone said I sometimes try to contribute something positive; a possible elaboration on one of their points, a clarification of an off-hand example if it's something I know well, an attempt to extend their argument to other territory, &c.

In cases like the fundraising one, where the problem is more individual misperception of group trends, we probably want something like an anonymous poll--i.e., "Eliezer needs your help to fund his new organization to encourage artistic expression from rationalists. Would you donate money to this cause?", with a poll and a link to a donation page. I would expect you'd actually get a slightly higher percentage voting "yes" than actually donating, though I don't know if that would be a problem. You'd still get the same 90% negative responses, but people would also see that maybe 60% said they would donate.