You could do this by adding an allow rule before the block rule like so:
appid: facebook (or how your block rule looks like)
Since PA uses top-down first-match visits to your business page would trigger first rule while other visits would trigger second rule.
Problems on the road:
* How are logins to facebook handled by PA (you might need to whitelist the login url aswell)?
* Even if you have www.facebook.com/mybusiness the other stuff on your page will be regular CDN stuff from facebook (which would be blocked by the second rule).
To fix that second problem you could create a custom app using facebook as parentapp (or web-browsing) and then add stuff like HTTP-HOST (*.fb.com or whatever the CDN is called) and HTTP-REFERER (^www.facebook.com/mybusiness). The problem now becomes that if any of your clients modify their browserheaders to include www.facebook.com/mybusiness as referer they will be able to access facebook (depending on which HTTP-HOST names you included in your custom app).
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