The real issue is more that those looser friendships are more common than you’d notice (because the NewsFeed algorithm hides them by design) and because they tend to be over-represented in the great diagonals and bridges, the connections that make that number (aka the graph average diameter) much smaller. Having many friends is, paradoxically, correlated with fairly strong weak ties on average; isolate people have few close friends, but they generally can’t trust their few distant friends as much. Having half of the network as those isolated nodes increase the average quite substantially if you exclude their distant ties.

There is also a non-negligible likelihood that those traversals go often through certain spam accounts that span strangers.