The pair, aged 17 and 18, used bogus profiles to contact mothers and offer them free baby clothes in a ploy to get their contact details, Derby Youth Court heard.
It is thought one of the teenagers was trying to get a baby so she could pretend it was her own, police said.
The pair admitted conspiracy to kidnap and will be sentenced on 20 May.
Derbyshire Police said the offer of free baby clothes was a "ruse" to get mothers' addresses and contact details.
The girls, from Evesham and Wellingborough, Northamptonshire, were arrested after the mother of a young baby in Normanton, Derbyshire, contacted police on September 21.
She told officers a young woman had been to her house claiming to be a social worker.
The woman told the mother the baby needed to be taken away for a medical check, but the mother became suspicious and refused.
A police investigation revealed the pair had tried to kidnap two other babies in the same month in Wolverhampton and Huddersfield.
Det Sgt Duncan Gouck said: "From the moment the older girl was arrested, she told officers she was pregnant but as time went on we realised she was faking this.
"We can't be clear of the motive in this case but we believe her intention was to get a baby which she would have pretended to have been her own.
"This case was unique to us in its nature and seriousness. We have never come across a case like it."
The girls were bailed ahead of their sentencing.
Two teenage girls have admitted using a Facebook "ruse" as part of a plot to kidnap three babies.