Being in web marketing, I get asked this question a lot. The simplest answer is it's all about customer service. Talk to your customers using the medium they prefer.
Social Websites Are All About Communication
I know many people that would rather chat on Facebook than email! It's becoming a very common method of communication. By having a presence on Facebook (for example) you also increase the likelyhood of being the chosen Real Estate Agent over other hopefulls for someone who prefers using Facebook to communicate.
Having a presence on social websites gives the appearance to your clients that you are technologically savvy. A percieved advantage is as good as a real one if it helps you get more customers, so even if you are not using these technologies effectively, the fact you are using them can be benificial. Put it on your business cards and marketing materials.
A common complaint you may hear about social websites is they are a waste of time, but they can actually be used to save time in some ways. They allow you to communicate with a large group of people with a single mouse click, they only thing you have to do is come up with an effective message to send out. :)
I hope this post will help some people who have been pessemistic about Social Media look at it in a different light, and maybe even unlock some of the potential power within. Thank you.