When housing became a problem, they put up rules about people owning land.

Now, for people owning a plot and having a house built on it, I remember the rule being 3 months. Example, I would have to use the house I built (actually go inside of the house) and that would reset your 3 month timer, this timer is also reset by people who use the same house that are part of your FC. If you fail to do this in the 3 month timer, the land will auto-relinquish.

I'm not exactly sure if this works for people who actually just -own- the plot with nothing built on it. It sounds like they bought it to re-sell it back at a mark up price, because land is at a pretty high price now (pending server).