CodeIgniter or PhalconPHP ?

if you reach such a dimension of traffic i think you have a complete other workflow, a bigger team and other problems ;o) 

Not really. The workflow depends on your own internal processes. You can have 1 click per day and a thousand clicks per day. Your business operations stay the same. Servers can go from VPS to dedicated servers or on-demand servers on the cloud. It's all the same at the day. As for bigger team, depends how badly the application is developed. If it's streamlined, then there are no issues.

To start an idea, a site or a platform, CI is for my opinion the best and fastest way. If needed you can refactor the site.

Exactly. I started on the project with CI 18 months ago. It was good enough to get the kinks out, prove the project and move forward with better resources. We started with just 1 vps, then a dedicated box, then experimented with amazon and now have our own cluster of servers.