Once a message is sent you don't know how it will be received. You hope you've hit the right tone but you know there's a chance it might be misinterpreted. What's more, as Muriel hints, you're aware your thoughts may be read by others besides the addressee.
And all that was when we were using the one-to-one format of the postcard.
Today, our new one-to-many means of communication (Facebook, Twitter, etc) can intensify those feelings of self-doubt. Take leaving a message on a friend's profile for the world to see. If it's left hanging without a thread it's easy to imagine it's been ignored by not just your friend but also by an unknown number of people who've seen it.
I suspect this has the effect of splitting messages into two broad categories. On the one hand, people aware of the risks in communicating play safe and reveal little. Neutral, innocuous remarks give the impression of not expecting replies. On the other, people in need of a bit of attention feel pushed into saying something quite extreme to guarantee a reaction.
One thing's for sure, I doubt Muriel would be able to comprehend that her "fine set of girls" is now on show to the world. Sorry M.
By the way, which one do you think is Muriel?