Linkbait is a bit tricky concept where a trendy content on your website which link to some other website (Because the website want to), But this is not because you ask them to link to.
1. Have you installed Gadgets / Widgets: Some peoples provides gadgets and Other app on the web - Once you install it they might be linked to their own domain without asking you.
2. "Powered by vBulletin" the word is linked to their own vBulletin Official website. This is also the form of link baiting.