Good to hear you've impressed so far!

1. You can't forecast the future, but what how stable is the Developer community? Is it a product to stay? And why?

The open source project (SilverStripe, sapphire and the 'core' modules) are maintained by SilverStripe Ltd (http://silverstripe.com/) which has been around for nearly 11 years now and has nearly 50 staff in NZ and Australia. On top of the official company the software is supported by countless partners all over the world - http://silverstripe.com/partners/. So it's not going to disappear any time soon.

a) How easy shall Silverstripe integrate with systems like MOODLE, OS COMMERCE etcetera?

Depends on your level of integration. If you want to have a unifed admin panel you *could* do it but there is quite a bit of work involved. If you want to run things alongside one another that is trivial. SS even exposes a Restful API for you if you need to include pages and menus on other sites from SS.

b) How easy shall Silverstripe integrate with other online platforms like LinkedIn, Facebook, Delicious etcetera?

Quite simple. I personally have got facebook connect working within SS (just the javascript API for now) https://github.com/willrossi/silverstripe-facebookconnect. Haven't tried those other ones but if there is a PHP library to connect with them, odds are you can get that working with SS.

Is there a possibility SS will be able to use modules from other platforms like Drupal or WP

No, all 3 systems are very distinct.

One other important thing is the newsletter module with campagne

There is a simple newsletter module available. If you need anything more advanced I recommend using MailChimp or a proper service like that.

I know there are a view, like from Drupal. How easily can i import those

You can import the HTML, CSS and javascript but the template languages are very different. You would need to replace and rebuild most of your Drupal theme with SS controls.

I have just had to go in the reverse direction - a new client wants me to manage a Drupal site.

As a Silverstripe developer, I'm now learning the meaning of the phrase "Drupal-pattern baldness". Silverstripe's clean separation of code, HTML design and client admin interface is wonderful by comparison.

Actually the unstable branch DOES work with 2.4.5!