I'm 35. I was smacked quite a lot. It got worse as I got older. None of my friends ever told me that they were smacked.
I was smacked on the back of my bare legs so hard that my dad's handprint was left with a white raised outline. The youngest I remember that happening I was 5 or 6.
I was hit with a hairbrush around the head by my mum if I squirmed having my hair done.
I was slapped around the face for back chatting by both.
I had my face squeezed so hard that my teeth cut into the sides of my mouth by my mum.
My sister was headbutted by my dad because she was rude to him.
My other sister had a dinner plate thrown at her head.
Another sister had her head pushed into the floor repeatedly during an argument.
It happened probably every week to one of us until I was about 16. Always as a punishment for a bad attitude or disrespect. Eventually, the police were called (by my parents because we ran out of the house to get away from them.) The police were very sympathetic to my parents and social services questioned me. They told me I was lying because my dad said I was and my mum agreed and that was the end of that. I left home. This was in 1999.
To this day my parents don't admit they did anything more than tap us when we did something dangerous like trying to touch a hot oven. Apparently, times were different then. Sometimes my mum talks about how she had no idea how to parent and she just did what her parents did etc. She blames my dad a lot. She likes to tell me that I'm lucky I don't have to smack because I have the internet to help me 
I don't understand at all. I think smacking (or worse) is only ever used to release an adults frustration or anger. It has never been an effective method of discipline. I'm so glad it is becoming socially unacceptable.