1. It's much easier to scale than it is to gain enough users where scaling is a problem.
2. The framework is almost never the culprit. For example, Disqus has 500 million monthly uniques on Django. 3. There are lots of ways to optimize system performance from caching, to optimizing queries, to refactoring, to upgrading ram/cpu, etc. 4. In most cases, only a tiny percentage of users will be on at the same time so several thousand users should be pretty easy for most frameworks to handle.
