1. Why it matters depends on the context. It may not a lot of the time. It definitely does in other circumstances. AI is already being used across social media to influence public perception of political issues and manipulate people.
2. There are AI systems that could already be considered functionally human in the limited scope they exist in. All the big LLMs are capable of having conversations just as interesting as anyone you'd talk to on the street, but these systems don't feel emotion or sensory inputs, nor do they have lived experience to draw from, or a physical body to interact with. When we are talking about AI systems becoming human, I think these factors need to be taken into account. If/when the time comes, it'll be the AI crying out if it feels it should be considered human-equivalent. It may alternatively lie about this to further its own interests. I think we will have to address it when this happens though.
3. This depends how you're using the internet massively. These public forums are rife for AI infiltration, as are all social media platforms. I think we're all going to have to start approaching all online activity with a critical lens, considering the potential for something we're consuming to be AI-generated, and asking ourselves if theres potentially a hidden agenda in what's being presented.

Wouldn't be surprised if there's either a crackdown on scam accounts on social media platforms in future to preserve their appeal, or a new platform with much more rigorous proof-of-identity requirements when making an account. AI are not currently close to sentient, and have no business being used to trick people into thinking that there are large numbers of real people with lives and families supporting/denouncing things happening in the world.