Everything is possible, it depends how skilled developer you are or you have. eZ Publish can be base for such systems as you can get some stuff out-of-the-box like content storage, caching, template system, etc. However that kind of sites are personalized and here you will need to extended eZ Publish. Also you will need most likely introduce new API for your users in order to enable posting via various channels, submit new items more easily, etc. For twitter like site you can take a look on the eZ Components example app based on the MVCTools component. http://svn.ez.no/svn/ezcomponents/docs/examples/applications/TheWire/
It is functional, you might extend it for your needs. It should be easy.
Can't say much without more information, because twitter & digg are totally different applications.
<b>Possible?</b>
Conceptually it would be possible to create such sites in eZ Publish and because the way ez publish works you would be able to add new features easily. Also performance thanks to stale cache of version 4.1 and with the correct architecture should be good.
Personally I would also mix it with eZ Components (perhaps even develop the site with eZ Components).
Depending on how much users the site will have you could came in performance issues so you have to investigate and plan performance/scalability even in the first project phase.
Twitter had (has) performance issues - anyway this is a in deep discussed topic all over the web.
<b>Reasonable?</b>
Perhaps it is...
<b>Easy?</b>
Should be easier than code it from scratch... Depending on the needs you can also mix a ez publish site with 3rd party services like twitter (for example there is a post to twitter extension...)