The difference is that one case is clearly explicit: when the ad says "Bob likes our pizza", Bob liking your pizza is an explicit endorsement. The other is more statement of fact: that Bob ate there. For all you know he could have hated it.

On the other hand, add the context of Facebook. What if Bob hit the like button in facebook? You could argue that hitting the button is an endorsement. Or perhaps if the ad is only displayed in a Facebook context where "like" is understood to mean pushing a button on a page, you could argue that the traditional meaning of "like" doesn't apply so it's not an endorsement.