Friendships as an adult

When you're an adult, you don't always think that friendships are necessary anymore because well.. you're an adult.  You're free to do as you please, come and go, whatever you want. 

But then you experience moments throughout your adult life, such as loss, grievance, new job, marriage, children, even a breakdown and you begin to realize how important those friendships we're back when.

I'll tell you one thing, making NEW friends as an adult, sucks!  You no longer just go with the flow, you're precise, picky and have a certain someone that'll click with your own personality.  Mom friends are ESPECIALLY hard to make and keep.

You look at profiles and think: 
Yeah! She seems down to earth.. not high maintenance, let's be friends.
Only.. she's thinks you're too low maintenance, you're not a coffee lover like her, or your kids are just too young.

I took our son to a place where he can see large vehicles, touch them and go play in them.  Most of the individuals that were there were moms with other moms.  There were a few families here and there but mostly it was moms.  
Do you have mom friends? Or dad friends? I'm curious as to how one meets and makes another parent-friend. 

We're all so diverse that sometimes it gets in the way.  

What are the secrets to making and keeping friendships as an adult?