Facebook plans to require its premium homepage ads to be in page post format, according to a document leaked to GigaOm. We’ve independently confirmed the news with someone briefed on the matter.

[Update 2/22/12 4:11 p.m. - A Facebook spokesperson tells us, "This a change that brands were informed of last month, in which Facebook's premium ad units that point onsite will now also be associated with a Page post on Facebook."]

The format change is not a major departure from what was previously available to advertisers working directly with Facebook, but it does show an emphasis on page content rather than traditional ad copy. However, the document suggests the company will concede to advertisers who want to run homepage units that look like page post ads without making posts to their fan page first.

Premium ads are homepage units that can only be bought by working directly with Facebook. These ads are meant for large brand building campaigns so they are sold on a cost per impression basis and give an advertiser 100 percent share of voice on the homepage, meaning no other ads run next to premium units. The ads are larger than those running elsewhere on the site, and they include more prominent social context — images and names of friends connected to the advertiser. This upcoming change is not likely to affect Marketplace ads (non-premium ads available from the self-serve platform and Ads API partners).

Premium page post ads became available in November 2011, but Facebook continued to offer its classic Like, event, poll and video ads. According to the document, the change will go into affect Feb. 29, the same day as the Facebook Marketing Conference in New York City. Here is a breakdown of which page post ads will replace the existing premium units:

The leaked document says ads that originate from page posts perform better than other types of ads:

• 40 percent increase in engagement
• 80 percent more likely to be remembered
• 16 percent increase in fan rate

That’s why it’s interesting Facebook says its team will create ads that don’t get organic distribution as page posts if desired. This allows advertisers to run messages that are only relevant to non-fans or a small subset of fans. It also allows for creative testing without spamming the wall. It seems in some of these instances advertisers could experiment with Marketplace ads before going ahead with homepage campaigns, but we assume Facebook wanted to be flexible and supportive of its high-paying advertisers.

These are the page post units that will replace the classic premium ads. Standard offsite ads will not change.