there are milestones in social communications, such as knowing how to play with other children, taking turns, listening, eye contact, etc. Some children develop those skills differently than others and it doesn't mean anything, whereas other children will have more pronounced issues.
My youngest DS has a speech disorder, but at 3 he had about 30 words, most of which were very badly pronounced. He was also late at socialising with other children, he would walk away from other children (apart from his brother and a couple co very close friends). At birthday parties or in large groups he would always walk away and play on his own or sit on his own. He didn't communicate that he was hungry or needed anything until he was about 3.5.
On a positive side, it doesn't actually mean that there are more significant issues, it's just one area of their development and it's good that you receive support now. DS' social skills really only started developing in Reception and he now talks about his best friends at school, he wants to have a birthday party and invite them all. School has been fantastic for him.
I have a very good book called Motivate to Communicate which gives lots of ideas for listening games and games that encourage communicaiton, both verbal and non verbal. It's on amazon and it's not expensive. It says it's for autistic children on the cover but really it's for all children who might have communicaitons problems.