This is a really great post from Eric Meyer. I too am frequently irked by Facebook’s reminders to tell me to feel a certain way on a certain day. The best examples are birthdays of people which I’ve met once or twice, but somehow we ended up being friends on Facebook.
The best takeaway of this though is to try and design experiences around how users are likely to be feeling at a particular point. For example error messages are not a place to make jokes…