Joanna Hi Joanna. Instagram is gaining popularity because it’s a newer, more visual platform that is appealing to a slightly younger audience right now. Facebook is almost becoming ‘old school’ now that everyone and their grandmother has an account. It’s not the cool place to hang out anymore for younger audiences and so over time you see user demographics changing and younger audiences jumping to new platforms (hence the rise of Instagram, Snapchat and TikTok).
It’s fine to post the same on both networks - then people can engage with your posts on the networks they choose to hang out on. If at any point, you see Facebook not achieving any engagement for you and your audience is on Instagram, you could decide to leave FB?
Facebook’s organic reach has been declining for years now, while Instagram’s organic reach has more potential due to hashtags and different features e.g. Reels.