The "recognized leader" must be one that the majority of the group approves of. A bus driver (leader of the bus) or a Facebook Admin could potentially hold strong persuasion over the group, but the group can elect and follow a new leader at any time. Imagine, on a bus, that the bus driver does something the people disagree with. One of the passengers stands up and says, "He can't do that!" If the majority of the people on the bus agree with that one citizen, that citizen now holds greater power to control or manipulate the mob than does the bus driver. To answer your question about how these "leaders" influence the formation of the mob, remember that the bus driver didn't create the bus and the Facebook Admin didn't create Facebook. They are still limited by the confines or structures of their environment. They have some semblance of persuasion, but the city (for the bus example) and Facebook (for the latter example) are ultimately the reasons for these groups forming in the first place.