I'm going to have to repeat "Ideas are worthless without execution". Current technologies lets you turn your idea to a product and launch in a very short period of time.
I'm not sure if you are aware of "Lean Startup" methodology but strongly recommend you get aquatinted with some of the principles. Web 2.0 companies like Dropbox, IMVU, Aardvark, KISSMetrics etc have applied these techniques to be successful.
You don't necessarlity need to share your idea with anyone but SHOULD build it and launch as soon as possible, your goal in the first month should not be to get lots of users but validate your hypothesis and collect data on usage (like if your users like your offering and wanting to come back)
I posted this comment on a different thread but re-posting here:
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We launched a product recently http://sociaholic.com and applied lean startup principles.
Problem Hypothesis: The existing Twitter to Facebook integrations and apps don't preview links/videos when publishing to Facebook from Twitter, thus by limiting the desirability to click on a link on facebook pages.
Execution: Design to launch along with a video in less than 2 weeks, the goal was to validate our solution has a market. We update your facebook page from twitter in nice way (previewing links, converting @handles, etc)
Validation: Our first week: Over 200 active users out of ~250 signups, conversion rate (homepage to adding twitter account) ~30%. 2% dissatisfied customers (5 out 250 people uninstalled)
The product has a much bigger vision and Customer Development is doing wonders and we plan to do this for every step we move forward.