Unlike the HTTP/1.1 protocol there is no method to host more than one ftp server on a single IP. HTTP/1.1 provides an additional transaction header, "Host:", to allow the server software to route the request to the correct virtual configuration. Currently this capability does not exist in the FTP protocol specification.
The only workaround to this limitation if address space is tight would be to host multiple servers on the same IP but different ports. however this is not a viable solution for a "normal" hosting farm because of the use of non-standard ports.
There is a draft standard draft standard under consideration with the IETF which extends and improves on the current FTP specification including support for a HOST command. However given that the IP crunch is coming from websites and not virtual ftp servers this is unlikely to be pushed through any time soon.