As @andy commented on your post it depends what kind of "Chat" you mean.
If its the IRC/AOL traditional style chat room that it is completely based on time of day, type of site, and many many other factors.
If your talking about the traditional website "chat" or commenting I would consider the following...
Reddit (A very popular site with a very engaged user base) on average receives about ~9 comments per post(2012-2013). For a site that is #32 in the world and #8 in the US that seems incredibly low. Before you think to much about that consider the vast number of submissions across the different sub-reddits and the number of posts that have 0 comments and the number of posts that have 1000+ comments, so 9 posts on average doesn't seem too far off.
I would say that if your videos are receiving anywhere near ~9 comments on average your doing GREAT and any individual video with 9+ comments probably have users that are "chatting" or engaged and interested in the content.