Y! Messenger monetized branded themes. National advertisers would pay for the experiences, users got to pick from branded backgrounds and emoticons. Chris Szeto, then Y! Messenger head, pioneered this revenue model. It was quite lucrative but then WebIM became a thing (a la Meebo) and the market shifted away from Desktop downloads. FF a few more years and we see Mobile eat WebIMs lunch. The cycle continues...

Thanks for the kind words about Meebo! I miss it a lot as well, particularly the people. P)