This must be the burning question that every homeschooling family answers over and over again and it is asked with a very real concern.
First of all, in my opinion, children do not learn their social skills at school - socializing with one age group only. They learn how to be socially acceptable from their parents, watching your every move. Children learn how to talk to the post office lady by watching you, how to treat their friends by watching you socialize with yours, how to empathize with someone healing a broken heart by watching you mend a splintered finger with a band aid and love.
Our behaviour around our children it vital, homeschooling or not, as to how our children turn out to be adults in this world. Teaching God's love for all people but speaking down about another race group for example, is sending them mixed messages and changing how they look at others - forever.
I find that in the homeschool environment my children get genuine praise when it is due and strict guidance when they need to be put back on the right path. This builds in them confidence to face situations that they will face later on.
An account I heard from one of the homeschool moms, when asked "How do they make friends?" answered "I get asked this over and over again, but now I just say - my children make friends just like yours, they walk up to a nice child they see in the park and say 'hello, my name is ...' and off they go. I don't see what makes it any different now that we homeschool?"
I must say here that you do need to give them opportunity to make friends and to have them over to play, as play is also a very important part of their development. I find that we actually have more play dates now that when they were at school, this is because we don't spend tedious hours in the afternoons doing homework as all our schoolwork get done mostly by lunch time. This will change the older they get, as more hours will be needed to put into their studies, but by this time play dates will be changing too. Now that we homeschool, I find that we have children over at our house more than they are invited out to play as parents think that perhaps I have more time to myself, but this is okay with me as I know where my children are and what they are getting up to.