Facebook has been telling marketers to expect an announcement at Ad Week in New York tomorrow morning. Word appears to have leaked out early. It will be partnering with market research firm Nielsen to better measure the impact of ads, according to the Wall Street Journal. Facebook has confirmed the story with us but says it will be giving out more details tomorrow.

This is another step in Facebook’s advertising system becoming more mainstream. More, from the report:

Under the partnership, terms of which aren’t expected to be disclosed, Facebook advertisers who are also Nielsen customers will be able to measure the impact of the ads they buy on Facebook through polls that Facebook will show its users who have seen the ads. Facebook will then compare the users’ responses to those of other people who didn’t see the ads and package the data for advertisers, according to the companies.

Facebook’s in-house brand and self-serve ads have appeared to be fueling the company’s revenue growth over the past year. The company is on track to make more than $550 million, largely from these sources.

The product will be called Nielsen Brand Lift, the report says, and “the companies also plan to launch a service that would give data about advertising on other sites.”