Contacts offline when they're not

For the last few days, all my contacts have been showing as "offline". I
thought this was a bit unusual as I knw at least 3 people who leave their
computers (and Messenger) on all the time! So I signed in to Messenger with a
different address, added one of my contacts, and they appeared online. I went
back to my normal address, and they were offline. So I deleted them from my
contact list, added them again and they reappeared! I did this with a few
more contacts, and now they are reappearing too. Any idea whats going on?