How opinion leaders are identified in social media? What indicators are used to identify them?

Question description

Influence impressions are primarily delivered by opinion leaders. Opinion leaders are defined as consumers knowledgeable about specific products, have a high level of involvement with such products and in general, they are social active and self-confident.

Questions

How opinion leaders are identified in social media?
What indicators are used to identify them?
In your opinion, why celebrities like Kim Kardashian are considered the most influential celebrities in social media?