It depends on a lot of factors. Every woman's motherhood experience is unique, as is she, as are her children. Some people appear to slide easily into parenthood and do it effortlessly, for others it takes everything they have, and I don't think there is a way to predict which way this or that friend will go.
Some children are easier to care for than others, and this is true whether they are babies or teenagers, so if you have friends who become parents of a baby that keeps them up all night, you will be seeing less of those friends than the ones whose baby adapts naturally to mom and dad's schedule!
Over the years, I have had friends who disappeared into parenthood, and what social life they maintained drifted to other people with kids in the same age group, so that all activities could be planned and centered around the children and their care, and I have friends who just sort of took it in stride, made little pillow forts around the baby on a bed or even the floor, and it slept through all kinds of noise, and grew into a toddler who was kept quiet and happy with a box of crayolas and some paper, or sidewalk chalk and a driveway to decorate.
And we have all seen people valiantly try to bring their kids along, with varying degrees of disastrous results.
Whatever the case, it is not that the new parents don't love their old friends anymore, and while you may not see them as often, you can get together with other friends and give the gift of a babysitter or have mothers take turns taking all the kids to provide the overwhelmed mother with an unencumbered day at the salon, the mall, or just sitting around gossiping like you have always done!