There is a lot of thing that come to play...What houston says about traffic=$$ is true to a certain point, traffic= $$ if you are planning on using advertisement as a source of income, but with little traffic you can also make money but that would depend on what you are providing for your members. I will give you an example, If I was a Dolphin expert, I would simply open a site and charge $5 a month for all the support you need. I won't need much traffic to make money. In this case this is an example see more of a niche. If your site is for social only, you won't make much, if your site is about "Georgia Collage students" with tutorial and study guides, you will make much more, "And that would still depende on your marketing skills"...I will tell you that to make my online is not just, "make a site" and make money, you have to plan the various aspect which needs to be considered on your plan...

Questions to ask yourself:

"What's the site about, who is my market, how will I promote, what is my cost, what is my timeframe, what will be my conversion rate, who Im i competing against, how is my site different from others and why would ppl chose mine instead, should I make income of advertisement or membership fees?

These are just some of the questions you need to ask your self before you even being to choose a domain name that way you know in what direction to begin....Hope this help, good luck..