Check out ProPublica’s website today, and you might notice — along with blog posts, donation buttons, links to special projects, and the kind of deep-dive investigative journalism that the nonprofit outfit is celebrated for — a new feature: advertisements. Starting today, the outfit is serving ads on its site to complement the funding it takes in from foundation support and reader contributions, its two primary revenue streams.

“This has been something we’ve been expecting to do for some time,” Richard Tofel, ProPublica’s general manager, told me in a phone call. “It was a question of when.”

ProPublica isn’t alone in venturing into the realm of dot-org advertising. A number of ProPublica’s nonprofit peers, California Watch, Texas Tribune, Voice of San Diego, and MinnPost among them, already run sponsored messages on their sites, served directly and via community partnerships and corporate underwriting. As Tofel noted in a blog post explaining the decision to take on ad support: “We’re doing this for the usual reason: to help raise revenue that can fuel our operations, promoting what people in the non-profit world call ‘sustainability.'”

The revenue raised, though, won’t likely be much in comparison to that offered by ProPublica’s other funding streams. The site had 1,300 donors in 2010, Tofel notes, providing $3.8 million on top of the funding provided by the Sandler Foundation; web advertising being what it is, the revenue that comes from the ads will likely be a trickle compared to the site’s donation-based funding streams. “Given what’s happened to web advertising in the last five years, on any kind of reasonable projection of the size of our audience” — though the number fluctuates, ProPublica currently averages a little more than a million pageviews a month, he told me — “we’re not talking about a great deal of money.”

So HuffPost this is not. And that will be true not only in terms of the revenue generated by the ads, but also in terms of their content itself. ProPublica’s ads will be served as part of the Public Media Interactive Network, a digital ad network — operated by National Public Media — that started in 2008 to sell remnant ad space on NPR.org and PBS.org, but which recently expanded to include nonprofit news sites. (According to this press release, Texas Tribune and MinnPost are also members.) The network sells packages; publishers can either opt into or opt out of running those packages’ ads on their sites. And while “we’ve looked at the range of their clients, and I don’t see any at the moment that we’d have a problem with,” Tofel notes — none of those “lose inches of belly fat!” monstrosities here, folks — ultimately, “it’s our decision about whether to accept a particular advertiser that they have found.” As ProPublica explains in its new Advertising Acceptability Policy statement:

First, ProPublica reserves the right to accept or decline any advertisement or sponsorship it is offered.

ProPublica will decline to accept advertising that it knows or believes to be misleading, inaccurate, fraudulent or illegal, or that fails to comply, in ProPublica’s sole discretion, with its standards of decency, taste or dignity.

ProPublica, like all quality publishers of original journalism, maintains a clear separation between news and advertising content. Advertising that attempts to blur this distinction in a manner that, in ProPublica’s sole judgment, confuses readers will be rejected.

It’s an expect-the-best/prepare-for-the-worst approach to ceding a bit of control over what readers see when they visit the site, Tofel explained. “You just want to leave yourself the latitude so that you don’t get into a situation that is uncomfortable — or that undermines, most importantly, readers’ faith in what they’re reading.”

And that transaction with readers — one that, ultimately, understands an audience not as an anonymous collective of eyeballs and click-givers, but as individuals and, in the best sense, message-amplifiers — will remain a constant even as ProPublica tweaks its revenue strategy. As will, Tofel notes, the outlet’s partnerships with other news organizations (48 last year alone!) — and its rare-in-the-media-world comfort with sending users away to other outlets, partner and otherwise. While, yes, the inclusion of web ads represents an interest in keeping — and growing — direct traffic to ProPublica’s own site, “we are not in business to make money,” Tofel says. “We are in business to make change. And that’s still very much the case. But we do need to come up with enough money to float the boat, not just today and tomorrow, but on into the future.”