Citing the partnership it signed with Microsoft for banner ad delivery as the change-point, ad buyers are complaining that social networking site Facebook is too restrictive with their ad offerings, according to MediaWeek.

Last September, Facebook lifted the requirement for registration that members be students, and membership has exploded from 13 million to 19 million by this Janury. But ad buyers say that Facebook isn't fully monetizing that growth. One concern mentioned is that the site withholds user data that could be used for better ad targeting. There's also frustration that Facebook doesn't offer many opportunities for rich-media ads.

The biggest concern, though, is that the deal with Microsoft has taken a large chunk of available inventory off the table. One Facebook marketing executive says that there were issues integrating Microsoft's ads with those on the rest of the site, but says those issues have been resolved.

For its part, Facebook says that while some advertisers have been turned down, it's largely because the proposed ads didn't fit the site's style. Instead of simply advertising, it seems, Facebook wants marketers to be more organically integrated into the site's community. One of the most popular tactics of marketers on Facebook has been the creation of sponsored groups whose members get access to content and information in exchange for joining.