By "social circle" I did not mean people I go out for drinks with - I'd call those "friends". I simply meant people I have something in common with, share the same values with, people I can relate to. If the candidate is older with kids (assuming I'm younger) - he/she should be someone I want to become in the future, someone I respect, someone I'd learn from. Race, gender, age, even education - does not really matter - as long as I'm genuinely interested in that person and feel like I'd enjoy working with them. After many years of interviewing and hiring at various startups this is more important to me than technical skills. Skills could be learned, personalities usually don't change.

I feel like programmer discourse goes in a circle every five years on agreeableness/human factors when hiring. You can say "rigid process and blind hiring!" And then you hire someone you knew was, er , difficult, and it sucks and they get fired and you have to adjust. Then on the other side of the spectrum, you have grumpy people pointing out that it turns out agreeableness turns out to mean coethnic drinking buddies, and how could you?

Why is that a problem?

I am more productive (and generally happier) when working with people I like, and can relate to. Communication is better too, which is pretty important.