I had some sites in mind so I'll just enumerate those:
Apparently they (eventually) considered this a major issue as well after some customer feedback, and thus Netflix will no longer split services, so there's obviously some strong reasoning against that approach.
In my opinion one of the best examples of multiple services. Stack exchange allows you to use one of multiple Open IDs to log in to their entire network and allows you to keep your profile information across all sites, and a nice tie-in with the gamification system allows your progress in one service to affect the others with effects such as updating your flair/profile with your accomplishments from other sites, the chat profile and lifting the "new user restrictions" on all sites after you get to 200 rep on one site.
To address potential privacy issues Stack Exchange lets you use different logins for each site if you want to, and doesn't automatically log you in to each site, leaving privacy in the user's hands to a reasonable degree.
Stack Exchange has some "unfair" advantages here though: All the sites are of the same format so the profile/ect data easily migrates, and all SE services are and always were under the exclusive control (not at the content level of course) of the same company, so more complicated issues such as absorbing old accounts to a new system and matching accounts on different sorts of sites isn't the problem it is with for example Google Services.
For long they've been the king of consuming other services and integrating them into the Google family of services. Google has turned their own account service into an Open ID, allowing almost all Google services to function exclusively via the Google Open ID.
An interesting exception is Youtube; because Youtube was an acquisition it was necessary to keep the old accounts distinct, so now when using a Google account to use youtube you sign in using "[email protected]" to avoid potential login conflicts, but your name appears in the service as it normally would.
Due to privacy concerns and the breadth of data Google has on it's users they've implimented the Google Dashboard to allow users to see or delete data from many Google services at once, which is an interesting way to allow some control over such a system.
These seem to be the key to all of these problems. Commenting on Blogs and newspaper articles used to be a royal pain, enter Disqus and now you only need one login to access a great deal of sites and maintain a certain "reputation" between them as your name remains the same and Disqus tracks your "likes." Facebook and twitter have also allowed one to access a great deal of services with relative ease and reasonably well known privacy implications. Stack Exchange has even created their own OpenID.