Facebook hosted an exchange between Michael Adebowale and an overseas extremist ahead of the murder of Lee Rigby, ITV News understands.
In the exchange Adebowale spoke of his intent to murder a soldier.
In a statement, a spokeswoman for Facebook said they took "steps" to prevent people using the site for terrorist purposes.
Like everyone else, we were horrified by the vicious murder of Fusilier Lee Rigby.
We don’t comment on individual cases but Facebook’s policies are clear, we do not allow terrorist content on the site and take steps to prevent people from using our service for these purposes.
Lee Rigby's stepdad and sister criticised social media companies for failing to notify the authorities of users plotting a terror attack.
ITV's UK Editor Rohit Kachroo looks at the findings of a report into the extent of intelligence agencies' knowledge of Lee Rigby's killers