1&2. They can message people via the network or via text messages (the former is more secure than the latter). If she's addressing multiple people at once, she can easily throw together a "group text" of sorts. She would need to know someone's name to address them directly, but you can certainly handwave usage of the network for easier communication without needing to make a new post every time.
2a. People are still taught how to write, albeit on digital/AR surfaces with styluses - though you're right in thinking that all signatures (and similar things) are done digitally.
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1&2. They can message people via the network or via text messages (the former is more secure than the latter). If she's addressing multiple people at once, she can easily throw together a "group text" of sorts. She would need to know someone's name to address them directly, but you can certainly handwave usage of the network for easier communication without needing to make a new post every time.
2a. People are still taught how to write, albeit on digital/AR surfaces with styluses - though you're right in thinking that all signatures (and similar things) are done digitally.