Gustav explained this 119 years ago.
Crowds (and communities) are irrational, illogical, and easily swayed by those with charisma making loud, absolutist, arguments.
Your're probably thinking "ha, yes, they are….but not me!".
Are you sure? Do you hold a believe that isn't support by evidence. For example, you might believe that community management is a rapidly growing field.
In which case, can you explain how many of us there are? By what % has the profession grown? What % of companies have hired one?
If the answer is no, then it's an irrational opinion you've adopted from the crowd.
Once we form this opinion it's reinforced by everyone in the crowd we speak to. This makes it seem more true than it is. It may be based on the flimsiest of anecdotal evidence ('look at all these community manager jobs posted on jobs boards!')
In itself this isn't terrible. Communities need shared beliefs and values. But community members take action based upon beliefs acquired from their social groups. This might cause them harm.
The bigger problem is countering these false beliefs. Evidence alone doesn't do the job. Does this grapth persuade you that community management isn't growing? I bet you're challenging the data in this graph (instead of finding evidence that trumps it).
Anecdotal examples and crowd opinion is stronger than data-driven evidence.
You can't tell members their beliefs are wrong. They don't want to listen to you. You can't give data-driven evidence. They will forever find the tiniest of flaws in the data to save their opinions. In fact, it's really hard to persuade people en-masse. The Obama birther movement is a great example.
However, you can reach out to people individually and make more rational arguments. You can try to build a relationship with these members. If they like and trust you, they're more likely to accept your evidence. When and if they express opinions driven by data, you can promote those opinions to the broader group.
You can challenge the leaders gaining authority through repitition, rather than validation, of their arguments.
If a community holds a false-opinion, simply giving them data or evidence that disproves it won't change their minds. You need to persuade individuals and promote the opinions of those individuals.