Guild leaders have been sending us a lot of questions in the Queue and elsewhere about how to keep their guild active during the down time between expansions. At this point many guilds have already cleared Orgrimmar and are either slowly working on heroic modes or stalled out entirely. It's not that anyone is doing anything wrong -- they're not -- it's just the nature of players. After things have been accomplished, why bother continuing on? This is totally a normal behavior and shouldn't be taken as anyone abandoning the guild or anything silly like that.*
So, life is slow on the forefront of raiding. What can you do? Change your game. Make the game not about killing pixels from this expansion, but about experiencing everything in a social atmosphere. I've had to make this transition a lot, and I've always found it works.
End current content raiding
Make an official end to the current tier of raiding and move to other activities. Allow for a small group to form that wants to keep pushing into harder heroics and what not, but don't make anyone feel like they have to go. And don't feel obligated yourself too -- it's okay in a guild leadership position to sit out and just facilitate.