I don't know about FCs, but they do lose their houses after being inactive. We've had quite a lot decay and go up to be rebought on Hyperion (which then get bought out again pretty quickly). Both my husband and I just bought newly freed up houses in the past week, a medium and a small. The houses you think are owned by inactive players are either still having people log in (remember FC houses just need one member to step inside) or they're just getting bought before you notice. The medium house next to mine I was sure was an inactive player, looked him up online and he's still in all i90 gear with a completely empty house, but nope see him teleport in front of his house occasionally so they may still be logging in.