If you log in with incognito mode, the cookies created in that session will be deleted at the end of that session (when you close your last incognito window) and will not be available to non-incognito browser windows.
Right, that's what I thought was the reason FB can't track you outside of Incognito mode, but if they can track you even when you're logged out of FB then it sounds like it's irrelevant whether you're using Incognito or not.
If I remember correctly, the way they track you is that they retain certain cookies that identify you even after you log off. But if you delete all the cookies you have from FB once and then delete any new cookies you get from them after each session (via incognito mode), wouldn't that fix the problem?
I suppose that they could also track you by IP address, but that would be pretty unreliable since most home users have dynamic IPs.
If you really want to avoid tracking, you can use a browser extension like NoScript (or the similar NotScripts for Chrome) to prevent the JavaScript code that puts their buttons on third-party web sites from loading.
