There is no such thing as 'normal' contact with adult children.
Some people will see an adult child daily, others do not see them for years on end. The same applies to telephone, Facebook and every other form of communication.
Once children grow up and are independent they call the shots and what those shots are, is dependent on the character of the child, how they perceive their childhood, which is not the same as how you perceive their childhood or how their childhood actually was, where they live, family commitments their spouse and so many other factors.
So, as I said there is no such thing as a 'normal' relationship.
It is obvious that you are unhappy about how little you hear or see of your youngest son, and that is never a happy feeling. As older adults, we just have to accept what our children offer. It seems that it is only your youngest son you are unhappy about, so make the most of your relationship with your other children and reach out to your youngest child, but you will have to accept what he offers. he clearly still keeps in touch with you, and many estranged parents would give anything just to have that contact.