...having a relatively unpersuadable audience is, I believe, common to all explicitly political media. Only people who like following politics would tune in to Meet The Press and people who like following politics usually have strong views about politics and are thus unlikely to be swayed by things they watch there. But there are more questions to be answered than "should I vote for the Democrat or the Republican in November?" Blogs are much more likely to persuade people on issues like "John Edwards or Barack Obama" or even more so "as someone who doesn't even live in Maryland, should I care about the Al Wynn versus Donna Edwards primary?" or "is the telecom immunity provision of the proposed changes to FISA a big deal?"
Blogs are a niche medium for political obsessives, so they tend to impact readers' opinions on questions that normal people just wouldn't bother having opinions about at all. That's not the same as saying that no persuasion happens and it's all preaching to the choir. It's more like the members of the choir talking about choir-related issues that others may not really care about.
For me, this means giving money to the congressional candidates Atrios tells me to, which I've done the past few election cycles, and figure to do into the indefinite future. I feel comfortable doing this because I've been reading about 10 posts a day from him for the past five years or so, and I know our views on the world are very closely in sync.