Frederic Lucas-Conwell conducted a study of Silicon Valley technical evangelists called “Technology Evangelists: A Leadership Survey.”
Here is the abstract:

The purpose of this study was to gain a clearer understanding of the relatively new phenomenon known as the “technology evangelist.” By our exploration, we aim to help readers improve their management functions, and to understand how best to integrate “evangelists” within their organizations.

In order to do so, we analyzed the roles of those who hold this position and leadership styles. Our research included surveying and interviewing twenty-nine technical evangelists worldwide from a variety of cultures and organizations.
Some general tendencies regarding the role did, indeed, emerge. However, we also discovered variable character, or personality, patterns among the participants. Therefore, we proceeded to examine the gap, between the role of the technology evangelist and the subject’s personal character.

We contrasted individual competencies with these character patterns, and created a grid to analyze their qualities of leadership. This paper includes our recommendations for recruiting, integrating, developing and managing the technology evangelists.

It provides good insights into what motivates evangelists and what makes them tick. If you want to learn about evangelists, check it out.