Proftpd fails to start if the /var/run/proftpd folder is missing (like, for instance, when it's mounted by tmpfs on boot). Proftpd's initscript should check the folder and create it if missing. sys-apps/dbus initscript already has such a line, so I think it should be valid and easy to accomplish.
Thanks for your report. A few days ago ProFTPD 1.3.3rc4 has been released and I
posted an ebuild patch and a new version of the ProFTPD init script in Gentoo
Bug #305343 which are solving this issue.
Thanks Bernd, fixed with new 1.3.3_rc4 just added to tree from bug #305343