Do not forget to consider what things might be worse if you don't skip. People tend to assume that not doing anything cannot make things worse, but it can.
As far as the number of friends whether you skip or not, I think you need to ask yourself, is my kid the type who will have a lot of friends or is he never going to have lots of friends and much prefers to have a few close friends instead. It may be that being in one grade is a better fit socially than another, but it also might not make a huge difference if it is the personality of your particular child.
I also second the Iowa Accelleration Scale.
