Facebook's plan to create a digital currency comes with one major flaw, according to an executive at blockchain start-up Ripple.

Marcus Treacher, Ripple's senior vice president of customer success, told CNBC earlier this week that a big problem with the social network's Libra project is that it's a "walled garden" — in other words, a closed system.

The term has in the past been applied to tech companies like Facebook and Apple in relation to the control they have over their software and apps.

Facebook's proposed libra token will be managed by a Switzerland-based organization known as the Libra Association, whose members include Visa, PayPal and Uber. The coin would be tied to user deposits in currencies like the dollar and held in a digital vault known as the Libra Reserve.

Ripple is known for its blockchain-based payments network that is used by some of the world's largest banks to move money across borders in real time. Blockchain, originally known as the network behind bitcoin, is a digital ledger that records data across a network of computers dispersed around the world.