Here is an article explaining how some Chrome extensions report on everything you do on your browser. What pages you open, what cookies are used, etc.

Now, if someone installed an extension called "Free Smileys and Emoticons" then we can't expect that person to understand the risks. But then there is an ad blocker extension, with more than 110,000 users. Suddenly some of the excuses for blocking ads ("it's about privacy!" or "the ads carry malware") are just bullshit because people don't even know what the extension is doing behind the scenes.

The equivalent of downloading programs from download.com and having all infested with that spyware they inject.

Interesting read. You should check all your extensions now.