This matches my experiences. People have become flaky to the point of absurdity. I don't know exactly how or when this started, but I suspect that cell phones have altered people's behavior in this regard.

Consider that before cell phones, you couldn't just send a text to cancel plans last minute - if you made a plan to show up, and you wanted to bail, you still often had to show up because you couldn't be sure someone would have a phone available. Someone not keeping their plans would cause concern. Now everyone's got a cell phone in their pocket and can send a message 5 minutes before a scheduled event to say they're not coming. It's become normal, and the more it happens, the more people will scale back on planning get-togethers because there's no way to depend on anyone showing up.

To be clear, I don't think it's cell phones per se that led to this, but the behavior of people with the ability to communicate with anyone, anywhere, instantly. People feel safer changing plans last minute because it's easy and they feel no consequence - only the poor friend who put the time, money, and effort into planning gets the pain from the fall-out. Everyone else just sends a text and goes on their merry way, oblivious to the fact that many people are doing the same thing and it absolutely can result in whoever's planning the event being left completely alone. I've seen it myself for parties, I've seen it happen with work-planned movie nights, and here it's happening to you, too. Something in the culture has changed, and I don't know how we can rectify it.