These days, you can hardly go to a website without seeing integration with Facebook–specifically, that near ubiquitous Like button.
But you won’t find it on the main Google page (although “sharing” on Facebook via Google Reader is still possible).
Interestingly, LinkedIn seems to have forged a partnership with Twitter–or at least seems to think that there’s not sufficient overlap between the two services that including a “powered by Twitter” doesn’t pose a major threat.
Again, no Like button.
The “cost” of omitting the Like button: making it harder for users to share their favorite stories on the world’s largest social network. The benefit: ostensibly keeping users on the “home” platform for a little while longer.
Are the costs greater than the benefits? In the Age of the Platform, who knows? Those looking for definitive proof can keep looking.