Nothing has to be done.
Abandonia is doing pretty well. We get 4 new games on the site every week, we've got an active forum with more topics than any single person can keep track off. We're getting a wagonload of pagehits. The bandwidth is being used by the gigabytes. So what if some of your friends/co-workers leave.
That's what happens in the real world as well. People switch jobs, they get fired, they're taking a sabbatical whatever. It's the same on the forum (only less dramatic and serious, and instead of a decent reception on which we can drink all we want we're stuck with some crappy farewell-thread
).