Many Covens have been established over the course of time. Most have had good intentions upon their creation, meaning when they started out they were striving to provide a place for those of common interest, teachings, and activity. Sometimes Priest/Priestesses do step out for days at a time to gauge the Coven activity in their absence. Is this a good practice? No not at all, to step out due to unforeseen issues in their personal lives is one thing, but to do it for the heck of it is absolutely retarded. Some Covens have very popular themes about them, such as Vampires, Dragons, Black Magick, etc. And these themes will draw the public's eye every time, though the down side is that now you will gain a lot of rift raft in the mix.
The best way to cut down on inactivity is to make a small list of what your Coven would deem acceptable, such as a requirement to post every so often, or log on every so often, etc. Set this list, then enforce it. Somme would say that this would seem like a control issue. But fact remains no one wants to put all their free time into teaching, and gathering resources for any set of individuals who stick around for 2 weeks then disappear for months. This is highly discouraging for teachers, and thus why some of us have an extremely hard time keeping good teachers on this site. Much less those who do remain having the desire to teach.
For those extremely large Covens sooner or later will grow tired of mass members but little to no activity. Just as you would weed out your forums, you must weed out your inactive members from time to time. No one is in any Coven because they have to be, they are there because they desire to be. Along with that desire should be the willingness to remain active. If they are not willing, then remove them. If you set the ground rules and enforce them no matter what, then before you know it you yourself will not be able to keep up with the threads.
Another way to keep up activity is to appoint a strong Council. Meaning members that are knowledgeable in the areas that your Coven stands for, also make sure they are helpful by answering questions and giving guidance. Never put someone in a Council position just because he/she is cool or your friend. This will under mind not only your position but the entire Coven system. Council should as the moderators of this site, they keep the peace and the ball rolling.
Keep in mind a Coven should not base their worth on the amount of members, but not its teachings & knowledge. "A Coven of 300+ is no stronger than a Coven of 13 if its walls are blank and leaders are inexperienced. " ~DH