Like others have pointed out in prior answers this looks like a mismatch between what users perceived when they clicked on your FB ad and what they discovered on your landing page. Closing this gap should be priority #1.
Talking to potential customers is probably the best way to resolve that.
Assuming these changes are implemented,
will you then be able to incorporate an "exit-intent" plugin that asks users "what they were looking for and why they are leaving"? This plugin pops up just before users are about to switch or close the tab/window in the browser. You can entice users to provide feedback via giveaways or coupons etc.
This will provide you a continuous feedback loop that help improve your landing page value proposition and understand your customers.