The popularity of Facebook is unquestionable. Personally one of the reasons why Facebook became so popular is because it was a place you could go that had minimal advertising and just information and pictures from your close friends and family. With Facebook becoming a publicly traded company there is a definite push to make Facebook more advertising friendly and this might mean that Facebook will become less popular.
I know from my years of experience in the marketing game consumers look for real estate when they are in a position to buy and when they are in a position to buy they like to look at all the options. There are always going to be dedicated websites to selling real estate and trusted real estate agent consumers will use to help them find the property that meets their needs.
I don't think social media sites like Facebook will have much of an effect when it comes to advertising real estate.