Disclaimer: I deliberately didn't read your description above to emulate the impression I'd get if I turned up without knowing anything about your site.
That said, I think you're focusing too much on what/how people post content on Wehgee, and not nearly enough on the benefits people can expect from sharing content this way. For example, Twitter's landing page says "Find out what’s happening, right now, with the people and organizations you care about", and Facebook "helps you connect and share with the people in your life." — both of these are very direct ways to introduce the product.
In contrast the way your describe yours seems purely functional: I know that with Wehgee I'm suppost to share stuff in 6 weekly topics, and that by doing so I'll be "shaping the conversation" but that has no relevance to me whatsoever and doesn't communicate a benefit to me of any kind.
Also, I think you need to be clearer about the distinction between a category and a topic. Initially I thought that the hashtags were the 6 categories, but it took me a while to work out that this wasn't the case, and that the two were separate. I think you need to first make it clear what categories of content there are, and then find a way to distinctly communicate what the current topic is for that category.
Another point: How do I know when one week's topic ends and the next begins? Is it on a Monday? Sunday? Can I revisit topics/discussions from past weeks?
I think there's some serious potential to giving people a framework of topics/categories to use for posting content, I just think you could find a clearer way of articulating the benefits of this approach.