My Piwik wasn’t tracking any traffic. I didn’t notice that it had stopped in July; I thought it was from the update to 0.8.
vipsoft helped me out and told me that Hostgator made some mod_security rule changes in July and these are the reason why Piwik doesn’t work on Hostgator shared hosting accounts.
If you are running your Piwik on Hostgator and there is no data tracking, this is why. Ask support to exclude/whitelist your domain(s) from these rules and you should be fine.