That membership size is still sustainable enough, for now. You should try and keep things ticking over just fine, but if the staff become absent for much longer, it may be time to migrate. After all, if they're losing interest, why should anybody else stay?
You could try and communicate with the staff. Search their nicknames on Google, and see if they are active anywhere else (you can try match their info up with things like age and location, to be sure). However it may be best if you take over a lead position before the community crumbles, as people will still have high morale and be more willing to continue over at the new community.
This is because people find it a convenient place to stop contributing any more, when their old community comes to an end and everyone is fighting anyway, people do start asking why they are still there, and move on to other communities. By taking the initiative now you can bolster the existing community and retrieve it, as well as being able to tailor it to your own ideology.
That said, you should make provisions for the site owners to return and should endeavour to reach a compromise if they do return once you've migrated.
