A better way to look at it is that social media connections are preposterously tenuous.
Stanley Milgram was a fucking hack but his "small world" experiment was basically mailing randos in Wichita and saying "hey get this letter to someone you don't know in Boston" with tear-off postcards along the way to document how many steps it took. That sucked too:
Shortly after the experiments began, letters would begin arriving to the targets and the researchers would receive postcards from the respondents. Sometimes the packet would arrive to the target in as few as one or two hops, while some chains were composed of as many as nine or ten links. However, a significant problem was that often people refused to pass the letter forward, and thus the chain never reached its destination. In one case, 232 of the 296 letters never reached the destination.
However, 64 of the letters eventually did reach the target contact. Among these chains, the average path length fell around five and a half or six. Hence, the researchers concluded that people in the United States are separated by about six people on average. Although Milgram himself never used the phrase "six degrees of separation", these findings are likely to have contributed to its widespread acceptance.
So really, Milgram was measuring "how likely are you to pass along a chain letter" while Facebook was measuring "how tenuous are your tenuous connections to your tenuous friends'friends'friends'friends?"
"Six degrees of Kevin Bacon" is another matter entirely as there are fewer casting directors in Hollywood than there are representatives in Congress and SAG only boasts 160,000 members (of which maybe 1% are noteworthy). Thus, the links between 1600 people who are cast regularly by 400 directors is very different than the links between 7 and a half billion strangers.