Facebook Is For Messagin' Not Just For Wordscrapin'

I've only recently started to avail of Facebook's instant messaging application. I found myself having a laugh with the delightful Miss E of Vancouver town as recently as late last night. It was nearly bedtime for me - but she was fresh as a daisy for sure.

I also had a brief interchange last night with Beowulf (we talked dogs and Ikea furniture) and tonight I've been discussing lipstick and tattoos with Mr Damien Mulley.

So tell me this, experienced peeps...

What is the polite way to break off a real time exchange, bearing in mind that I'm only supposed to be on the computer a few minutes at a time (hah!) as I have dishes to wash, old ladies to attend to, cakes to bake, books to read and husbands to smile at invitingly?