This is the first time in more than a decade Facebook has ditched the well-known reference, which emphasises the fact it doesn't cost to become a new user.
Business Insider used internet archive Wayback Machine, which reveals that the company switched the slogan at some point between August 6 and August 7.
The publisher is awaiting comment from Facebook about the change.
Though Facebook does not charge its users, it has taken advantage of using their data and selling it to companies to help them with direct marketing products to certain audiences.
"Facebook is not free nor has it ever been," lawyer and digital law expert José Antonio Castillo told Business Insider who apparently was the first one to tweet about the change.
"Facebook's currency was and still is it's users' personal data. It's never been free, though, because data is worth a lot of money."
As to why Facebook decided to alter its slogan without any warning or explanation, Castillo suspects it could have been sparked by a directive that European Parliament passed in May that recognises that exchanging data is actually a form of payment.
Although Facebook's original slogan was "It's free and it always will be," its terms and conditions contradict that. In its Platform Policy, Section 7 (Things You Should Know) Clause 11 warns that: "We don't guarantee that Platform will always be free."
