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As you know Facebook is a huge platform with millions of daily users. These users use abbreviations to communicate with each other. Being a Facebook user, you have seen the ISO acronym on the platform. The ISO can have various meanings but the most common meaning of the acronym is "In Search Of". It's a way for users to express that they're looking for something specific, whether it's a product, service, information, or even people. You'll often see ISO used in groups, Marketplace listings, and comments.

Source: https://getassist.net/what-does-iso-mean-on-facebook/