Don't be evil is (or was?) the informal corporate motto of Google and such was Android in some kind. One of the reasons Android became so successful was the reason that it was, more or less, open to everybody - handset manufacturers and carriers as well as services and software developers which means different apps and services can play nice with each other. Think about the sharing feature in the picture gallery where every app, which is able to post photos to some services, can be linked-in. It's the same with different services under the "Accounts & Sync" menu. Typically, Android phones come with pre-supported services like Google, E-Mail (POP/IMAP) and Exchange accounts but 3rd party apps can also be found here after installed.
If accounts are configured from Accounts & sync, in most cases they appear in the address book of the Android device to show unified information of contacts, for instance latest Facebook updates and tweets but not with Google's very own Nexus devices, which is somewhat strange. While 3rd party accounts, like Facebook or Skype, appear after the respective apps were installed, it's impossible to add and create the respective accounts to link them to the address book. The account setup opens but hides away immediately without any further notice, leaving the user pretty much alone and clueless.
However, it seems that it's a question of which 3rd party services are allowed because Twitter works well and syncs contacts and tweets as Google+ does, even if Google+ appears under the Google account settings, as all Google services do.
Also Twitter and Google+ status updates and links to the respective profiles are provided to synchronized contacts so this feature is definitely supported on vanilla Android devices, like the Galaxy Nexus.
But Facebook and Skype (as well as most likely other services but this is not confirmed at this point) are not supported, which might be seen by Google as direct competitors. At least is Facebook a threat to Google+ while Microsoft's owned Skype is a threat to Google Talk and Google+ Hangout. However, it's not that Skype isn't integrating itself well because it's offered as a dialing option, if a number is dialed straight from the address book.
According to our information, this isn't a bug at all, neither with the Nexus S nor with the recently launched Galaxy Nexus but Google has done this limitation on purpose, but only for its own Nexus smartphones. All other Android devices - even with vanilla Android versions - are supporting 3rd party accounts and sync, giving the user the freedom of choice. That's a kind of weird behavior, to restrict API access for 3rd party apps and services and it doesn't fit into the philosophy of not being evil. Is Google really in the need to keep it's competition locked-out from its smartphones and how does this behavior makes sense on devices which are seen as the developer reference platforms? Me and our sources might be wrong and maybe Google hasn't done this on purpose, maybe it was just a - never fixed - mistake because Google has not too much experience with the Android OS? Well, sounds pretty unlikely.
Bottom line? Google Nexus smartphones might be the most "original" Android devices but they also come with the most original limitations, manufacturers like HTC, Samsung, Sony Ericsson or Motorola wouldn't dare to do! So users, which looks for the widest flexibility might better pick-up a device from one of the manufactures above, even if a major OS upgrade can take a month or two longer.
We've contacted Facebook and Skype to get their view about the issue above. In the meantime, if you find other incompatible apps, please feel free to report them in the comments bellow.
UPDATE [29.12.2011]: While I'm still waiting for a feedback from Facebook, Skype was so kind to answer my question about the Galaxy Nexus issues: "We are aware of the problem of Skype not properly syncing with phonebook contacts on the Android Galaxy Nexus. We are currently working on a solution to the problem and plan to release a fix for this with our next release. We apologize to all users of the Galaxy Nexus for the inconvenience." While this doesn't explain what the root cause is, it's at least good to know that Skype is aware of the problem and will release a solution hopefully soon.
Cheers ~ Arne