Networking Process - Who do you stay in touch with?

Hi guys.

I'm curious when networking how everyone chooses who to stay in touch with. Say they're either alumni or people who you were connected to by others, how do you stay in touch with everyone? I'm asking because I realize coming out of undergrad (now 2 years out) there were a lot of people I spoke to that I didn't stay in touch with, some I look back and see they didn't engage much and we didn't have much of a dialogue. Others were helpful (had calls and a good amount of emails) and left their current firms and I unfortunately don't have their contact information and it's been a year+. 

How does everyone go about this, say you haven't had a few phone calls or talked to them every few months with advice on a new role if you're young and sending congrats about a deal/happy birthday/follow up every x months when you have something. How important is your out of undergrad network? My thoughts are there are a group of people you will talk to frequently in your industry during this time period that will grow as you progress with more reasons/frequent contact.