Google sites is a pretty useful and interesting. It allows you to really create a site how you want it to be. It offers you vast opportunities to be as creative as you want to be. You can really be the creator and decide what's included and how. However, networking is not a simple as it is with the following sites (below). You have to send it out where you want it to go.
Plaxo, in my opinion, is sort of the bridge between facebook and Linkedin. It is more professional than facebook, but less than linkedin. It offers more opportunities for networking. It seems very user friendly as well. I think it will be a site that I will use more in the future.
Linkedin has not really appealed to me... yet. it seems somewhat difficult to use, maybe I just haven't spent enough time on it yet though. It seemed very restricting with it's options and choices, but it is a very professional site (the most professional, IMHO). I don't see myself using linkedin at this point, but this may change and the site could change too, to make it more user-friendly.
Out of the 3, I like google sites and plaxo the best. They all have their ups and downs, but I do like the user-friendliness of those two best. Again, I am still learning all of this, and this could all change with time and experience.....