Brian Tolkin, who built UberPool into one of Uber's most popular products, is leaving the ride-hailing giant after nearly five years.

On Monday, Tolkin tweeted that it was his last day at the company that he joined in 2012 right after graduating college.

Tolkin may not be a household name like Travis Kalanick, but his role at Uber was to build one of its most important products. When he first started in 2012, "there was hardly the thought of UberPool or even UberX," Tolkin once told Business Insider.

UberPool started as an experiment by Tolkin's team to see if Uber could bring down the price of rides below UberX.

"We noticed that a lot of [rides] were starting and ending in very similar places and people were moving around in their cities in very similar ways," Tolkin said at the time. "Would it be possible to put two or three of those people in the same car and drive down the price even further?"

Now UberPool is one of the most-used products and it's grown to account for more than 25% of rides in cities where it's launched. Tolkin, as its first product manager, has always been one of the biggest cheerleaders for how shared rides could change transportation around the world, but there's no word as to what he's doing next.

"Brian has been instrumental in our efforts to use technology to put more people in fewer cars," said an Uber spokesperson. "We are grateful for his hard work over the past 4+ years, and wish him the best in future endeavors."

Here's what Tolkin posted to his Facebook page about his departure: