I took over an RP that a GM quit but I had his permission and the players agreed. It's not the biggest or busiest but it's still going, even survived the guildfall, mostly because the players loved the characters involved and the story we are building together. 

I think the key is to get permission or if the GM just vanishes, then put it to vote among the players or the people interested. If you have to make a new thread, then don't just cut and paste it, make it your own.