Human beings are not pack animals! Yes, we are social beings, but our primary and most important arena for socializing is the home. Little ones need mama - and dad and any other siblings - and occasional visits to this or that person and into the larger world - but until they can really play properly, ie have started to develop a sense of "I", they really don't need much. Even then, home is the most important place - and should, in my experience, be the main arena of life until about the 9 year change. And if there are no siblings? I still think little ones do not need much other than their family.
Pretty bold, eh? This is my observation of children both who have this kind of life and those who don't. And it's my observation of my family, too.
This does not mean no play dates, being isolated and avoiding park days! It's a question of age and a question of balance. I think that until about 3, there is no need for more than occasional visits to play or regular play dates - yes, more can often work, but I would keep a close watch on the child's behavior. Many "issues" and "challenges" which arise at this time are nothing more than symptoms of a child with an overly busy, overly stimulating life.
From 4 on this can gradually increase. And (List Member X), looking at what you wrote, that is a pretty busy schedule to me! And it might be fine if the rest of your family life is very ordered, predictable and has strong rhythms and that, most importantly, you are not feeling stressed out by going here and there. With such young children, that could really effect them. (and no guilt intended!)
And I know that many of us go out a lot with our young children because being home all the time can at times be so lonely, boring and stressful (it can be - let's be brave and admit it!!) So sometimes we also have to do things to help ourselves as mothers so that in balance, the children will benefit (happy mom equals happy home). There's no one recipe for this. But I do know from many consultations with homeschoolers, that the more they cut back on commitments which took them out of the home, the more perceived problems with the children seemed to disappear - even if they didn't change anything else. That has been quite amazing and right across the board, something said by people with very peaceful calm home lives and those with hectic lives and no sense of rhythm.