Consignments made out to be genuine brands are being sold by people using fake profiles on buy and sell pages for the south-east of England.
Cheap tobacco purchased by BBC South East was found to contain double the lead content and a third more cadmium than genuine products.
Facebook said content in breach of its commerce policy was always removed.
The cut-price tobacco and cigarettes are being sold through the social media site's Marketplace pages and offer hand delivery.
Undercover reporters arranged to meet a seller in Tunbridge Wells and purchased items marketed as Mayfair cigarettes and Amber Leaf rolling tobacco.
When analysed, they were found to have higher than normal levels of lead and cadmium - both toxic metals that can cause damage to the liver, kidneys and brain.
Jon Griffin, analyst at Kent Scientific Services, said: "We are not good as human beings in getting rid of contaminants.
"So the likelihood is you are going to be taking in more of those contaminant metals and they will remain in your system in organs, in the brain, in other organs in the body and you will not get rid of them.
"Over a period of time there's a bigger potential health risk."
Manufacturer JTI, which makes the real Mayfair and Amber Leaf products, confirmed the items were counterfeit.
In a statement, the Weybridge-based company said: "Over the past year, JTI's actions have led to the removal of just under 3,000 listings relating to the illegal sale of its brands on [the social media site].
"These products have an estimated street value of [more than] Â£400,000."
Facebook said it investigates any reported prohibited activity and it had removed the post advertising the counterfeit tobacco.
Almost identical adverts for tobacco have since been placed on other buy and sell pages, using different profiles, the BBC has found.
You can see more on this story on Inside Out South East on BBC One at 19:30 BST on Friday 31 March, and later on the BBC iPlayer.
Counterfeit cigarettes and tobacco are being advertised and sold through Facebook, an investigation has found.