Most people over estimate the minimum number of people needed to make a site look busy. Its around 20-30 [1]. Can you get about 20 zealous people to participate on your website everyday?

[1] I've moderated private invite only forums with 20 members that generated 12-15 threads everyday! And yes - these threads were busy. 20 people were easily spending a total of more than 10 hours on the forum everyday.

Action Summary:

- Find 20 people who like your concept and invite them to participate before you launch. Make them your beta testers and ask them for their input. Give them non-monetary rewards (tshirts etc is a good idea). Let these folks populate your site and make it look busy.

- Then on launch day, have a strategy in place that does generate 500-1000 visitors per day. Thats probably about $50-100 in ad expense per day if you don't have a viral aspect to your social website and your users don't drive traffic.