Best way to collect Rails application usage statistics

I have a Rails application where users can set up their domains and publish content onto them. I need to collect public traffic statistics such as pageview count etc. One good example of this feature is flickr usage statistics i can see as a customer.

The question is what is the best way to collect usage information. Should it be done by parsing log files or should it be collected and stored in database at runtime? Is there any tool or Rails plugin that already provides this?

This solution should scale well, even with thousands of domains and millions of pageviews in month.