Christina Royal commented: "Staying at an airbnb this week. On our honeymoon. Smh (shake my head)".
Another Airbnb host, Jen Hunter, said she has been doing her "utmost for two years to treat my guests with kindness and respect".
However, with no way of knowing if the confession is legitimate Hunter commented again: "Pondering further, I wonder if this postcard was sent in by a hotel owner to try to damage Airbnb's reputation & is completely made up."
While Facebook user Lucio Rubino said she was sure the secret was "posted by a bed and breakfast owner that's trying get Airbnb to lose business."
The comapny has said that it takes privacy issues extremely seriously and have a zero tolerance policy for hidden cameras.
Airbnb allows people to rent flats, rooms or beds in one of the listed host's properties.
It has over 1.5 million listings spread across 191 different countries in 34,000 cities, Airbnb says.
This is not the first time the website has been caught up in controversy. The Mail Online reported last month how an Airbnb guest, who rented a bedroom in a house, caused £7,000 worth of damage after she used the home for a porn shoot while the owner was away.
Landlord of the California home, Sharon Marzouk, posted before and after photos on Facebook of her dirtied property.
She found evidence of a porn shoot, including a sex toy on the bed, a bag of condoms, a checklist of sex acts for clients watching her videos online, and used tissues.
An Airbnb spokesperson has commented that there is no evidence to support the claim.