I always build one for clients too, but honestly, the effort split is heavily skewed. Few things to consider based on running analytics across both platforms:
1. Why bother with a Facebook page at all?
It still serves as a public-facing business asset. If someone searches your brand on Facebook (which still happens, especially in older demographics), not having a page looks amateur. Plus, Ads Manager requires a page for running any paid campaigns - even if you're targeting Instagram placements.
2. Optimisation priority
I optimise posts for Instagram first - that's where the engagement metrics actually move. The same content gets cross-posted to Facebook via the native sharing tool or a social scheduler. No extra editing, no custom headlines, no tweaked CTAs. The Facebook feed is essentially a mirror. It rarely drives meaningful traffic, but it doesn't hurt to have the content there.
3. Tracking nuance
One thing GTM highlights clearly: Facebook's referral traffic is almost always lower quality. Bounce rates higher, session durations shorter. Instagram's traffic (even from Stories) tends to bring warmer leads. But I still fire page-level analytics through GTM for both properties - comparing the channels side by side is useful for the client's monthly report. If the Facebook numbers stay flat for three months, I'll recommend pulling spend entirely. Until then, the page stays live, but barely optimised.
Basically, build the page, throw the same posts at it, and move on. The real effort belongs where the audience actually scrolls.