HN user
Heh, I just realized I never answered the concurrency issue. The all-time peak for concurrent users was about 45. More typical is 15. However, there is a live audio stream associated with the site which probably tends to encourage more concurrency.
I think that this is somewhat dependent on the sort of site you're looking at. Total user engagement and usage patterns are going to be different on a news site is going to be different than on a social networking site.
In any case, on a fledgling social media (i.e. focus on sharing/remixing audio/video/images) that I built for a client, the current user registration is just shy of 2,000. The number of users who have posted anything is ~700. The number of users who have posted in the last 30 days is ~350. The number of users who have posted 3 or more items in the last 30 days is ~100. The number of users who have commented on a post in the last 30 days is ~150.
Would calling your friend instead of using Twitter been a disaster? It seems like a broadcast medium like Twitter was completely inappropriate to use in that instance. Contacting the one person who you wanted to reach directly would have made a lot more sense. This is the sort of mentality that turns me off to Twitter is that it's the end-all, be-all of communication. Use the right tools for the right purpose.
I often subscribe to the comments feed for the article (if one is available). That results in a lot of comment feeds, but they usually peter out after a few days (and I clean house in terms of RSS subscriptions every once in awhile anyway).