Definitely not the case, and I wish this tired cliche would go away already.
My homeschooled sister is far more social than I am (I did the normal school thing), although we're both fine at it. Homeschoolers probably have a lot more realistic socialization opportunities, as it's all based around common interests and skill levels, just like the real world, rather than being packed together with a bunch of people whose only trait in common with you is their age.
Stereotypes like this are broad generalizations and generally not representative of the facts... sort of like saying that all Blacks are criminals, all Muslims are terrorists, or that Gays are child abusers - none of which are generally true.
Most of the homeschoolers I have known have excellent social skills, and further, even the few "creepy" homeschool kids that are really awkward socially tend to grow out of it completely within a couple of years of college.
Also, you get to interact with only about 20 other random people of the same age with the same skill set (by definition). For 10 or so years. Very realistic, totally happens in real life.
I mean, schools are terribly inefficient at teaching skills/knowledge, but they are clearly even more terrible at teaching social skills.
They all seem like pretty good social skills to me.
There is a lot of benefit to the singularity of purpose at the same time there are a lot of side effects. To give you one example, homeschooled kid lacks good social skills, which are more important to success in life than mere intelligence or knowledge.