Here's the business promotion guideline as ironed out with the IRS:
There is to be no promotion of business or anything included in a wiki that is written by a for-profit business that would help promote the business. There may be pages on businesses which may include information such as hours, location and services. However, the pages may not include promotional material, advertising or prices. Each page is open for all in the community to contribute to and a variety of viewpoints, both positive and negative, are expressed by different individuals. Critical, many-sided and open commentary, discussion and opinion regarding local for-profit business community is intended to educate the public and consumers, promote neighborhood cohesion and lessen neighborhood tensions, and combat community deterioration.
The above is written in a specific legal language. The above (mostly the first sentence) is what we base our organization-level decisions on. But, again, it's in specific legal language that was written in a series correspondences with the IRS. And there is more to this: for instance, if content is educational in nature then it doesn't have to obey by this guideline.
Our Community Guidelines give the human-understandable side of this. And explain what this means for your wiki and your community. Our business promotion questions page answers some specific and common questions about this guideline.
This is here for reference purposes.