"Initially it was mostly coming in via email which we would reply to, but we've grown so much that now the more common thing is you set up a series of discussion forums in which users bring up various things that they think are important to change or modify in some way.
Users talk amongst themselves about things we're doing poorly or could be doing better, and then we're able to observe that interaction. It proves to be a very kind of efficient and interesting and useful way, nowadays, of digesting that feedback.
The other important aspect that you might not imagine initially is that all of the feedback is coming in as 'voting with their feet'. We just watch how people are using particular categories.
If we see that, 'oh users want to do this and we're not currently enabling this', then we try to code up some changes to better enable them to do whatever that is."