There's a pretty active Twitter community in this part of North London with some very prolific tweeters, including HOL so its quite possible that the HOL stream is at the top of the hill (although I'm a bit worried that if you zoom right in, it might just show me sitting in my living room :))
There are a lot of people that tweet in and around this area, so add in all those and it's not such a surprise. Simon for example has done over 9,000 tweets! I'm not saying that he is entirely responsible :) but I can think of lots of other folk that tweet daily in this neck of the woods.