Katz and Lazarsfeld way back in 1954 within their book "Personal Influence" claimed Opinion Leaders should be
"spelled with a little 'l'. As everyday influentials, they are ubiquitous"I take this to mean they could be anyone or everyone; with each person applying their own influence to their own sphere of contacts (at the time, this was generally by word-of-mouth or by simple one-to-one communication methods).
However Katz and Lazarsfeld also refered to some Opinion Leaders as "Great Disseminators" (a wonderful term that). This categorises people who have an important forum— national or international— and who are respected and listened to by a number of people.
Perhaps these early thoughts on an individual's social graph and the different scales of influence aren't too dissimilar to what we're now trying to do on a global scale and in the digital domain.... I for one whould like to know who our Great Disseminators are these days.