I think it depends how much you value your friends.
It's easy to get subsumed in baby and toddler world and forget you will ever have time to yourself. It's also worse for many women as their husbands/partners don't appear to be true partners, and they end up as default carer for the children all the time, weekends, evenings, whilst their partner goes out doing their hobby or coming home late from work.
The women I know who have the best friends and also have children all work f/t, and I've concluded that it might be because this group don't get stuck in the mum=household entertainer/dad=can't even care for his own children model. They are also used to going out and about, even staying away from their children, for work events, so the idea of a weekend with the girls is not horrifying to them, and they know their partner can do the sole childcare on weekend (I have friends whose husbands have never ever taken the children).
The busiest mums I know have time for friends, they invite them over for a quick pasta on a school night, to watch a match or social event at home, and it's always casual. I also took a week off when mine were about 3/4 and went on a tour to see all my good friends, with dad staying at home. It was brilliant! I also see friends when I travel for work, or when we are driving past. I email a lot, text a little bit if nothing else will do, call when I get a moment. Those moments are more and more now the children are heading into their teens.
Now the children are older, I'm glad I made the effort, I don't feel lonely or like I have no friends now the children are getting older and wanting to go off by themselves, I have lots of social opportunities/friends/people to call on in a crisis, but it's down to effort and having a supportive partner. If you don't have that, trying to maintain friendships is almost impossible.