If you count as an "English speaker" anyone with strictly greater than zero proficiency in the language, I'm sure the total number of "English speakers" is much greater than 1.5 billion.

Alkanadi:Good point. Maybe, the best selling point is that Esperanto can be used in almost every country whereas a national language is restricted to a locale.

I doubt there's a single country where you can communicate effectively with more people in Esperanto than in English. Geographic restriction may be a shortcoming of every natural language compared with Esperanto, except English. Even Spanish and French could be exceptions, too.