It's simply hard to plan. Before kids I'd typically meet up with friend around 8 or so maybe 9. Now bedtime rules my evenings. When my kids are asleep I'm exhausted. Most of my friends evenings are just starting at that time! (lol) and I completely understand. The other thing is I can't go out and get drunk or party because being hung over with a 3 year old pissing the bed after they crawl in to sleep/cuddle with you - nothing better/worse.
It's simply hard to relate. I have some very good friends who we've stayed in touch. I'm forever grateful for them. But when you're out and about and you meet a random person and try to strike up a friendship say at a conference. The second I mention I'm a dad I feel I'm relegated to the back of the bus.