You only have a few options. You shouldn't, and shouldn't be encouraged to, ignore absent FM's. In some way the staff needs to be told who is and isn't doing the job. Just waiting around until someone doesn't log on for 6 straight unbroken months isn't working. You can't post it in general discussion because people will suddenly become very offended - you can't use ask to talk about it because it isn't documented and likely only a few will see. Your only option is to use the complaint thread.
Not only is it important that the staff know who is and isn't interacting with their FM enough, it's a valid complaint. I'm certain they have staff who try to keep other staff up to date on those sorts of things, but a perspective from within the faith should be valued.