One aspect of the social proof provided by other true believers is that once a prophecy fails, they become VERY serious about prostletyzing.
I have not subjected it to serious scrutiny, but if I recall, the proclaimers book talks about a renewed effort to spread the truth. These renewed efforts always coincide with a major failed prophecy.
The reason is because whenever a prophecy fails, it is a serious challenge to the tb's faith. They need to ensure their are many others who believe as they do. The fact that their are all these new converts despite the prophetic failures helps gird the tb's faith.