I would say though that I think your articles and lectures could do with a bit more organisation. In both cases, the user would benefit from posts having tags denoting subject content, and from a search facility so that I can find things that specifically interest me. If you have five hundred videos about mathematics, finding one on Analytic Topology, say, by scrolling through them would be a bit painful. I also think it would be good if the lectures had some way of tying subsequent videos together - it seems a tiny bit messy to have 'Cosmology - Lecture 1', and 'Cosmology - Lecture 2' as separate entries. I'm also not certain that you want to limit lectures to being videos. A well-written explanation of a broad subject is sometimes better than a video that I can't search or skim through - and I think it serves a different purpose from the articles. It's also easier for your community to add written content than videos.
Hope that was useful to you - but obviously take it all with a pinch of salt. You know your strategy better than some guy off the internet who's only spent ten minutes on your site :)