FB has a captive audience, that is aging with facebook and will continue using it.
It is clear why FB purchases IG / WA. They are paying money to capture audiences with money earned from FB.
Going ahead, the cycle will repeat. Once the teens in IG grow into adults, FB will use money from that to buy / make another platform.
In this context, the metaverse makes sense. It is a virtual world, where new SM platforms are churned, with all future populations being part of one or more platforms. FB wants to be the owner of all the platforms.
From a business point of view, it makes more sense, in that, FB can now target ads across platforms, meaning, they target the same number of individuals, but with ads being channeled through different platforms, the ad density comes down, and the feeling of "the feed is all ads" might reduce.