Sounds like inet.d, or CGI behind a webserver.
[1] http://0pointer.de/blog/projects/socket-activated-containers...
[2] https://developer.atlassian.com/blog/2015/03/docker-systemd-...
Obviously this is tightly locked down with cgroups.
As for having more users than sockets, is that really likely, considering there are 64000 available ports?