Everybody has facts that they can't help but bring up during conversation. The Twitter user I frew up :( prompted people to share those facts, and started the conversation. 

"Basically everyone has a 'John Lennon beat his wife' style fact," he said, "that they can't stop themselves from sharing if anything even tangentially related comes up in conversation." 

 Twitter user no more docile users shared the fact that there is a drawing of a dick on the moon. Out of respect, we're going to go with that as the baseline for "irresistibly sharable fact."