Finsta is a shortened form for Fake Instagram, a slang term for an Instagram account used by young and adult teens to hide their personal details. Usually, people create Finsta to post inappropriate content for their main Instagram accounts.
You can set it public or private like a normal Instagram account, but these Finstas are private accounts with only 10 to 20 followers. For instance, many celebrities with verified Instagram accounts have their Finstas. They usually maintain a secret account for their personal use and even add private content for close friends. You can share funny content on your fake IG accounts and even add a funny user name for your account.
Example
Here’s the best example for Finsta, Bella Hadid created her Finsta account last year. She changed her name to Rebbeka Harajuku.

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