They won't be forced to disable their ad blocker. Their ad blocked just won't work on Facebook's ads because the ads will be served from the same place the Facebook content is.
Since legalese needs to accompany ads, what's to prevent blockers from blocking certain elements that contain it? Just serving ads from the same domain can't possibly be the only "solution" FB is relying on.
Not just the same domain, they'll likely serve the ad assets from the same location as the standard Facebook assets. So images from the same place they serve profile and photo assets. Javascript will likely be a single JS file with both the Facebook JS (required for FB to work) and the ad JS. The individual ads will likely be within elements that change id and class so those can't be blocked either.