While Instagram is owned by Facebook, not everyone uses both social networks.
you might’t do it by signing in to your Instagram account on the web at Instagram.com.
These instructions apply to iOS and Android.

The screenshots show the Instagram app for iOS.
Select theprofileicon from the bottom menu bar.
Tap themenuicon in the upper-right corner of your profile.

TapFacebookand put in your login details.
To share your Instagram posts and stories automatically to Facebook, selectStart Sharing to Facebook.
If you want to leave this disabled and enable it later, chooseNot Now.
When you create a new Instagram post and reach the caption tab, chooseFacebookto post on both platforms.
(An additional popup may appear asking if you want to automatically share all posts.
ChooseNot Nowagain to keep it manual.)
On the Facebook tab, your Instagram account is connected to your Facebook profile by default.
you’re free to change this so that posts are sent to a Facebook Page you manage.
To select a page, chooseFacebook Profilein theShare tocolumn.
A list of Facebook Pages you manage appears on the next tab.
Tap any page to select it so you’re free to share your Instagram posts to it.
However, it’s possible for you to choose to share only posts, only stories, or both.
snag the buttons besideShare Your Story to FacebookandShare Your Posts to Facebookto turn them on or off.