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If it's the bounce rate and not the website or landing page visits, then it's your content. You need to rewrite or reword your content so it's more engaging and interesting. Structure it so the most important, interesting information is in the first paragraph, but leaving them wanting more. You should also be making sure that each one of your Facebook posts accurately describes what's on the page. If you feel like you have to jazz up the description on Facebook, chances are the copy itself won't keep anyone's attention.