I think kb is on the right track as far as collecting all the metrics on the site. But to spin it a different way, what if there was an actual real world cost given to the uses of the site. Say you take the cost of bandwidth and then you divide that by how much each individual user is drawing from the site. Then you take the cost of storage and you divide that by how much each individual adds to that storage by adding content, comments, etc. It would be interesting to see a little ticker on my feed somewhere that tells me, as close and specific as possible, how much I as one user costs the owner to keep the doors open. This info would only be visible to each person. No one could see other users usage and dollar draw. As a user that has been here for a while, I would love to know how much I am costing Hubski, and would definitely be more likely to give money - and likely even more than I cost. You could take this as far as you wanted. Say you took all the time that was spent to code the back end, upgrade hardware and what not, well that could be a line item too. The more users on the site, the more these costs are divided. The more active someone is, the more they cost. Payment would not have to be required, it would just be a "hey, fyi, your 50 posts today cost us $.00065 in storage.
Another idea is to have an actual page that you could go to with advertising on it. It wouldn't be on any other screen but one. It could look like "feed - chatter - global - tags - community - badges - accounting" or something. I click on accounting and it's just a stream of video ads or something. Also on that screen, there is another ticker that tells me how much the site is earning by me watching these ads. Say I wanted to earn Hubski 50 cents one day because I saw that that was how much my draw was that day for using the site, I could watch ten ads, or refresh my screen and get more ads or whatever. Mind you, I have no idea how ads work or give money.
Edit: My first point reminds me of the new Aphex Twin album cover, where he breaks down where all the costs of his album went to. I know for example that .00041 quid went to tracking and monitoring illegal download links and that .00113 quid went to advertising in Spain: