"Potential generally refers to a currently unrealized ability."
So the potential would be all Twitter users. I'll use http://www.statista.com/statistics/282087/number-of-monthly-active-twitter-users/ for the current potential audience.
That would be 307 million users.
Don't know if you know what I'm saying,
but whenever I follow a person/like a tweet,
some other twitter-rers will follow me, and they are mostly the ones who own a similar twitter account to those that I follow.
How do they even find out? There's isn't a feature on twitter that could offer this info.