All those free tours are fake. First they are not led by professional ones (a professsional guide has got a card and you can ask to have it shown up). Therefore you can get a lot of misinformation and you wil not be able to get into some places where only real guides can take you. Of course IT IS NOT FREE as mentioned. Those "guides" have to pay the site through which you found them. And what about those people who have been living in Paris for 2/3 years pretending they know the city well...What a joke!
You'd better ask a real guide who is not going to lead a chock-a-block group and whose visit doesn't cost 12 euros per person!More