It really depends on the situation.  Do they have cousins near their age nearby?  Are they involved in an active youth group, scouts, community sports, etc.?  Is there a large homeschool community in the area?  If there are more kids in the same boat, it can be easier for them to make friends even if they're not in school. 

My niece is an only child and was homeschooled for a few years.  It was right for them for those years, but I was glad for her sake that she went back to school so that she could make close friends her age.  She didn't have cousins nearby, and since everyone in the neighborhood was busy with school activities, it was kind of rough for her.

If I had an only child and I really wanted to homeschool him or her, I'd really look into homeschool coops.  (I taught foreign language at one for awhile and felt that it was the best of both worlds.)  The kids meet once a week and learn things that parents often have a hard time teaching like foreign language, advanced science, etc.

I hope this helps a little!

 We did a co op for a couple of years. Did art, music,  basketball, ice skating,dancing. A lot of those subjects. Kids loved it. Gave them some social time.  Learned things that I had no patience to teach. Babs

my youngest son is quite a few years younger than his siblings so it's like he is an only child . I have homeschooled him his entire life . I will say that one of the challenges is keeping him interested in what he is learning because I am the only person to bounce ideas off of . He does have quite a few friends though and we have never been in a co-op . My best friend and I live right next to each other and both homeshool . This year I am teaching her kids ( plus my own ) science and my son will be doing a financial class with her kids .