Chrome or Safari? If you're using Chrome it's likely a dodgy add-on that's synced with your Google account. You can try check the extensions or unlink your profile/browse incognito to verify if this is the case.
If it's Safari then beats me if MWB came back clean. Worth installing an adblocker to see if the problem persists.
I can confirm that Taboola is used on io9. It's not you.
However, Taboola and Outbrain are both despicable. I've added /etc/hosts entries that work very well for me since AdBlockPlus seems to think they are acceptable to display. Here's my /etc/hosts if you want to use it. Granted, it'll only work for your Mac, so if you browse from your phone or tablet, you'll need to find alternatives.