free site offer their subdomain for free for a reason, members promote it for them and they get all the traffic not you, a domain can't be compared with a subdomain, once you get your own domain you can create unlimited subdomain as many as your hosting provider will allow.
If youre developing a web site for people to visit then having your own domain will reward your effort, especially if your site becomes popular, but if you just want a place for friends and family to hook up then the social sites offer a better solution.
i was like this before i thought hypermart.net and freeservers.net have saved me a lot of money for not registering my own domain but instead in the long run I have wasted time promoting both of them since they get all the traffic and promotion and also the ads.