We use it here at Cloudflare on every single machine as part of Workers. So that’s two major hyperscalers running large RAM multi tenant workloads.

Jemalloc may more recognition in the broader community, but the largest workloads seem to be running mimalloc / tcmalloc (I don’t know what Facebook uses internally). The libc malloc probably has even more users than either as it’s the default allocator for iOS and Android.