Short answer: Drupal is well suited to build something like that, especially if you are willing to integrate your app/logic into Drupal as a suite of custom modules. The other way, integrating Drupal into an external application, can also be done, but will give you more friction, as Drupals architecture is pretty much geared towards being a framework in its own right.
Longer answer: I have a pretty much opposite opinion/experience compared to Palantirs. I've been working almost exclusively with Drupal for a year now, in the context of two fairly complex/'enterprisy' projects (after several years of 'on the side' usage for smaller things). While I agree that it imposes some rigid rules (but not limits!), I consider this to be an advantage, as those rules give a clear guidance and provide proven ways on how to do things. The three parts Palantir mentions are good examples for this:
- Menu system - Provides a well structured and effective dispatching mechanism that is easy to extend with your own stuff, while giving huge flexibility to tweak/manipulate existing/default paths. (Note that 'menu system' in Drupal denotes the whole topic of managing your URL space, not just the subset of 'visible' menus that is usually associated with the term)
- Forms API - A declarative approach to web forms, with a well designed processing workflow and a whole lot of built in security features that you would otherwise have to take care of yourself. Also highly extensible, with straight options to adjust/extend already existing forms on demand, add new validation rules to any field or whole forms, multi step forms, javascript based form adjustments, etc.
- Translation system - This is pretty complex, simply because internationalization is fricking hard to do. But it is built in, again giving clear guidance on how to do things in order to work in a generic way (though there are problems with quite some contributed modules that are not using/supporting it the way they should).
I could give more examples for parts where I appreciate the 'rules', but this post is getting long already, and I still have to cover some downsides ;)
So to sum up the positive part - if I where given the rough specs you posted, I'd say 'no problem' and go with Drupal, being confident that it would be a solid foundation for the custom parts, while providing all the 'standards' like forum, blogs, twitter/facebook integration and many, many others in the form of already existing solutions (even though those might need some adaption/tweaking).
Downsides: As always, there are flaws, and some of them are substantial, depending on requirements/circumstances.
- Learning curve - Drupal is quite complex, and 'grokking' its concepts takes time. 'Playing with it for a week', as Palantir suggests, will certainly give you a general feeling/broad impression, but it is in no way enough to allow for a serious judgement of its pros and cons, as those will only surface while coding in/for it. So if you are already deeply familiar with an established web development framework, this might be an issue. If you have to learn one anyways, this should be less of a problem.
- Database restrictions - As of Drupal 6, database support is MySQL or PostgreSQL only, using a Drupal specific 'abstraction layer' (which obviously isn't one ;)
Drupal 7 will move to PDO, which should (finally) end this questionable state.
- Test/Stage/Production migrations - Parts of Drupals 'out of the box' flexibility are due to many things being configurable in the administrative backend, which implies that many important configuration settings are stored in the database. This makes migration of data and/or configuration between several instances pretty difficult/tedious, once you left the (early) stages of development where you can get away with complete dump/restore operations (see e.g. this question & answers)
These are the main ones for me, but you'll probably find more :)