I'd say the spectrum of Python web frameworks looks like this

web2py -> Django -> Turbo Gears -> Pylons -> Web.py

On the left you have more fully integrated frameworks that do more out of the box (and essentially make more choices for you upfront). On the right you have frameworks where you do most of the work yourself. These frameworks eliminate some of the boiler plate of writing a web applications and give you the responsibility of handling the rest.

Which one to use depends on what you're looking for.

There is no reason any of the above frameworks can't handle a "digg effect". Just throw servers at it.