If you're too busy to take 10 seconds to send a text, then you should really reassess your life.
I'm quite happy with the way I life my life, so feel no need to reassess it, however forgetting to respond to a sms invite doesn't mean my life is so worthless that I need to reassess it. On the flip side I send thank you's after my Engagement Party, Wedding, Baby Shower, and DS's first birthday, I'm a pretty decent (but unperfect) person living a busy life.

The courtesy goes both ways - don't harshly judge a person because they forgot/were too busy/ to respond to an sms, because on a crazy day they want to take 10 seconds to sit on their bum and catch their breath rather than thinking about person X's tupperware party. You never know whats going on in a persons life that makes them too preoccupied to reply.