Often, a stranger would come up to me and said "Hello, Mr. Tan. Nice to see you".
I could not recognize him and wondered if he was a former colleague, neighbor or business associate.
I asked him, and he said "I followed you on Facebook".
It
would have been better if he had introduced himself properly, and do not presume that I know his background. It would only be fair to me.
A 76 yo person sent me a private message to suggest that I should contest the presidential election. He introduced himself as being 76 yo and a complete stranger to me. It was proper for him to introduce himself in this way.
I find it easier to reply to a person who made a proper introduction.
I often get private messages from strangers who asked "Are you free to chat" or "Tell me about yourself". I delete all of these requests for being presumptions.
The girl from Angola send me an email and a private message from Facebook. She explained in fair detail about her family circumstances and requested for a donation that would allow her to study a course and get a job. She sounded sincere. I asked a few follow up questions and decided to help her in a small way (smaller than what she asked for).
It is important for a person to make a proper introduction before engaging with another person.
I hope that this advise is useful for you.