With friendships, there has to be a mutual amount of flexibility, esp. if we have children.
With good friends, you know an excuse is genuine. I've had to cancel with my dearest girlfriend a handful of times since having Kiddo, and she's also called me that handful of times with "I have cramps and hate everyone, so I don't want to go out tonight."
When we have that certain closeness, we can give our friends the benefit of the doubt because we know they still adore us even if they are stuck at home because of the husband being at work or because they have cramps or whatever. It doesn't sound to me like this piece of the relationship is there.
It's okay to let friendships go. I wouldn't say friendships are a dance, but more like the tides... at some point, one friend or the other may feel like they are doing 'more' of the work tending the friendship. If that feeling is felt by both persons involved, and is mutual, then friendships can withstand those tides because the burden of effort is shared.
Only you know how the burden of effort feels for you. So, if it feels like you are making more of an effort, try to see it objectively. It was obviously a pretty tough day for you today. Friends do that to each other--occasionally. Only you can decide if you want to put forth more effort or not.
If I did have a conversation with a friend like this, I might start it with an open-ended question: "Hey, it's been a long time since we've been able to get together... how've you been?" Sometimes, people have deeper problems they are not dealing with on Facebook (which actually shows good boundaries), or you might discover that you are right, and there is a lack of effort on your friend's part. I'd try to listen with an open-mind as much as possible, and to think hard before trying to 'fix' things.
To be very honest, if someone told me that they'd like to see more effort on my part as a friend, I think I would feel pretty defensive and it likely wouldn't be progressive to the conversation. This is why I suggest an open-ended question. And if you decide to let the friendship go, I don't know that an outright conversation needs to happen. At that point, you do follow their lead and go see them when/if it's convenient for you.