The cross-site trends are easily explainable- the same people use both sites. One site picks up something, the other follows shortly thereafter. I've even seen

The cross-site trends are easily explainable- the same people use both sites. One site picks up something, the other follows shortly thereafter. I've even seen comments replicated word-for-word between the two sites. Which I'm pretty sure isn't a sign of astroturfing, as it would be much too damning.