The totals I provided from the Facebook Power Editor are the ones given when you select all of the targeting options together, which is where the numbers from t

The totals I provided from the Facebook Power Editor are the ones given when you select all of the targeting options together, which is where the numbers from the article come from. If I select just Instagram then users in United States 18 - 24 is 28m and users in United States 25 - 34 is 34m. So it's not a clear overlap, it could be users with Instagram account not connect to a Facebook account or have no Facebook account at all. Instagram does allow up to 5 accounts to be signed in at the same time from a single device.