The DOI links aren't working so I can't read the study but I think there's a simple explanation, that also belies the headline. It may be that the typical number of close friends is now 0. But as people no longer even have any concept of close friendships, they're considering 'lesser' relationships as close friendships, because it's all they know.
This is made even more likely if they didn't define the term and allowed 'online friends' to be counted as 'close friends.' And I strongly suspect this may be the case since the graph shows a major inflection point in the increase of friends being 2007, the exact year when Facebook started becoming massive.