Facebook has temporarily stopped granting new memberships in one of its most important advertising programs — the so-called Preferred Marketing Developers badge system — in order to change it so that it focuses on advertisers and agencies who buy paid ad media on Facebook.

The message is clear: If you want to play with Facebook, you must now pay for the privilege — in the form of ads.

Separately, we noted earlier that Facebook wields a big stick when it comes to advertisers who don't want to follow the rules: They get banned, ultimately.

The policy change was articulated in an email to new potential Facebook PMD clients, which was obtained by Business Insider.

Previously, the program, which serves advertisers who want to have a presence on Facebook and learn more about the social network's capabilities, was open to companies that only wanted Facebook's free pages for companies, or those who provided analytic and measurement services that did not include paid ad buys.

But, "the new badge criteria will include a focus on paid media," according to the email Facebook is sending new potential members of the PMD system.

In a statement to Business Insider, Facebook said there was no strict link between PMD membership and paid ads:

There is no specific requirement that all PMD's run ads, which this article seems to indicate. Additionally, we are still badging on all other fronts, including insights, pages, and apps. What we are working on, just as we do for brands and agencies, is ensuring that the PMD ecosystem is comprehensive in its knowledge of the entire Facebook marketing platform versus a smaller subset. The intrinsic link between paid, earned, and owned media on Facebook means that PMDs must have a deep knowledge and understanding across all of those areas to ensure marketer success. Many of the most successful PMDs offer comprehensive Facebook marketing solutions, through creative partnerships with other PMDs or by offering multifaceted tools.

The new version of the PMD program, which will go live soon, will require advertisers and their agencies to buy paid media on Facebook in order to qualify. Those who do not want to overtly buy ads must demonstrate that they have "a Facebook holistic strategy and influence your clients on media spend."

The email is mostly likely to affect companies which offer their clients Facebook analytics, management tools and dashboards but don't prioritize paid ad buys.

Here's the text of the email:

Dear applicant,

We would like to thank you for applying to our Preferred Marketing Developer Program.

If you have been in touch with a PMD Team representative, please ignore the rest of this email.

If you haven't, we would like to apologize for the delay in our response.

The reason for this delay is that we have been reshaping our PMD Badge requirements and application process. Our goal is to make the PMD Program and its members even stronger by raising the bar.

This should go live in the next few weeks and we will let you know via email then.

In the meantime, here are the things you should know to get prepared: