To begin building an ad system Costolo turned to a physicist named Kevin Weil.

A longread on twitter’s history of monetization:

Weil had joined Twitter in the spring of 2009. His resume was standard issue for a Silicon Valley high flier: undergrad at Harvard (math, physics), physics doctorate at Stanford, and abandonment of plans to be a researcher in order to join the tech circus bustling all around him. After working for a couple startups, he agreed to head Twitter’s very small revenue team, at first trying to develop a paid analytics system for companies that wanted to use Twitter to track events and consumer sentiments in real time. When it was time to build an ad product in 2010, Weil’s five-person team was the obvious choice. The technical lead on the team would be Alex Roetter, a former Googler.

via How Twitter Found Its Money Mojo — Backchannel — Medium.