Key is efficient caching. So there are more seeds than leechecrs. This is also true with Freenet and one of the points which I really like Freenet & GNUnet versus other protocols. Older versions of GNUnet didn't have cache eviction at all which I found to be major longterm issue, but they fixed it after discussing the topic. Btw. Bittorrent could be also modified to provide caching, but currently it doesn't and that wastes a lot of bandwidth. Tor doesn't use caching either, and wastes even more bandwidth.