It's considered poor taste to talk about your numbers if you're a blogger or pod-caster, so while I'm thinking about mine, I'm going to offer some rough data points and talk about other people's numbers.
A high quality, and reasonably high circulation, but pretty average science fiction magazine has an audience of about 10,000 per issue.
I think the top end of academic literary magazines have audiences of about 5,000 with most of them closer to 1k or 2k.
The minimum bar joining the federated media ad network is (or was) 250,000 page views a month. Figure that 250k page views, works out to an audience of at least 100k but probably less than 200, depending on a lot of factors.
The average, but established, mid-list podcast gets about 4k subscribers.
I think TWiT podcasts (mostly weekly) get several hundred thousand subscribers/downloads, but not more than say, half a million.
I heard a while back that boing boing got 3 million visits a month. Its probably more now.
The New York Times has a daily circulation of 1 million, while the chicago tribune has a daily circulation of a bit more than half a million. The Wisconsin State Journal (the Madison, WI paper), has a daily circulation of just under a hundred thousand.
I've also heard it said that about 1% of podcast/blog readers can be expected to buy stuff/donate/give feedback. In my experience this is probably true. I think, it doesn't scale well for more popular blogs, so that more than 1% of Stephanie's readers comment and buy her books and donate to knitter's without borders. But it's still probably not more than 10%.
I'm not sure what any of this means, and clearly four thousand people buying a hardcover book is different than buying a daily newspaper, or subscribing to a monthly journal, or just subscribing to a free podcast or visiting a website. But having some kind of concrete set of numbers is helpful and reassuring when I'm thinking "is it worth while."