I believe this is an even bigger market than the one Mixpanel was previously tackling. There are innumerable UGC sites online today, and they all want to offer

I believe this is an even bigger market than the one Mixpanel was previously tackling. There are innumerable UGC sites online today, and they all want to offer analytics to their customers. For every one of them to build their own comprehensive web analytics system is just silly. Until now there was no product that solved this problem for them. If Mixpanel's product becomes sufficiently powerful and flexible, it can become the standard for "analytics on someone else's site" just as much as Google Analytics is the standard for "analytics on your own site".