[wisty]

Focus on the SMALLEST problem you are solving. People don't join facebook to upload gazillions of photos, or message their friends each second. They join because they need a way to contact people without tracking down email addresses. Then they upload a photo because it's kinda cool, then they get hooked on the ecosystem. But that comes later. DON'T SELL THE ECOSYSTEM TO NEW USERS. Sell a small solution (which doesn't have to be anything too special), then let them scale their behavior up. The ecosystem is what makes you rich, but it's not what get's the first few logins. The whole ecosystem just confuses new users.

Oh, and remember, there are lots of people who don't even know what a URL is. SO worked because lots of JoS / Coding horror readers trusted it enough. They also know what tagging is - most of your readers are more familiar with phpbb style indexes. Nobody looks at the tag cloud unless they have already know what they are looking for.