Follow recommendation concerns, from a single-user perspective:
How exactly is this going to work with federation? Will users only see recommendations of other users on the same server as them, or will it recommend users from other servers as well?
If the former, it's going to actively hamper any single-user and/or small instances' discoverability.
If the latter... well, your instance still has to know of remote users to recommend them. Nearly useless for those of us on single-user instances.