Sorry about your miscarriage. 
I am an only daughter, of an only father, with an only daughter.
Honestly, don't overthink it.
I am a bit antisocial, but that's not because I'm an only child. It's because a lot of people irritate me, I have a very busy working life, so my idea of heaven is locking myself in a room with a book and NO OTHER PEOPLE! 
Dd is the most sociable butterfly I have ever met. That's not to do with her being an only either. It's just the way she is. Her best friend (who has siblings) is a shy timid girl who can't look other people in the eye.
For every sociable extrovert child with siblings, there will be one without. For every shy retiring only, there will be a party animal. (I am suffering on 4 hrs sleep because of dd's pyjama party today for example)
I think the only thing while they are little, is possibly yes, you might have to be a bit more proactive at organizing having friends round and going to kid related things. But only until they get to school, then it's all taken out of your hands anyway. (thank God, softplay and park stuff, along with playing with glittery shit is my idea of hell on earth!)
People inevitably come up with the (rather bizarre) "what if something happens to your only child?" argument (the deputy head of dd's school told me that, just a few months ago "oh have another child, you must, what if...." I pointed out that a) I am 49 next week and have been menopausal and without a period since Christmas and b) should I take a lover on as well, in case dp goes under a bus tomorrow? 
Something I also notice, is that siblings are not very often "best friends". Of course, sometimes they are, but I think so many children are "forced" to play with little brother or sister (especially when one of them has another friend round) that it can turn into resentment. Dd's friend is 4 yrs older than her brother, but I always invite him too, as I know it would be frowned upon not to, and when dd goes round there, he is always involved too, and his sister, at nearly 12, rants in private to dd about how much she hates it. She is also now old enough to have worked out that it's just an easy gaff for her Mum, her daughter has a friend round, and she gets to offload the little brother too. Job done.
DNA is random clumping of scientific thingies.