Former Google exec and the cofounder/CEO of RSS service Feedburner Dick Costolo is Twitter’s new chief operating officer, we’ve heard from multiple sources. Costolo, who sold Feedburner to Google for $100 million in 2007, left Google in July. We’d heard he was looking to start a new company, but obviously Twitter swooped in and grabbed him.

Steve Gillmor is going to love this, of course, since he proclaimed that RSS was dead and Twitter was the new messaging protocol bus, or something to that effect. “Rest In Peace, RSS,” he wrote, saying “It’s time to get completely off RSS and switch to Twitter…All my RSS feeds are in Google Reader. I don’t go there any more. Since all my feeds are in Google Reader and I don’t go there, I don’t use RSS anymore.”

Santosh Jayaram, Twitter’s existing head of operations (and also from Google), will presumably remain with the company and report to Costolo.

Costolo, who is also an early Twitter investor, is someone who has actual experience building scalable infrastructures, which Twitter sorely needs. The company hasn’t launched any new features in recent memory, and continues to have regular downtime. In fact, Twitter’s inability to build features and keep the service live is a serious competitive disadvantage. Costolo can presumably fix all that.

Twitter is actively hiring more senior people, we’ve heard. In July they hired Alexander Macgillivray, Google’s associate general counsel for Product and IP, as their new General Counsel.