Answer by Bret Taylor:
Very few people used it anymore, and most of the engineers who knew how to keep things running had moved on to other projects or companies. At some point, the technical burden of keeping the service up was just not worth it.
I think Facebook was amazing for keeping it running as long as it did. They kept it up for no other reason than respect for its small but passionate base of remaining users. Few acquirers would do that, and it something the entire team genuinely appreciates.