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What does "along" mean here?

The whole phrase "along comes" simply means "comes", but is an emphasized way of saying it. Therefore, "along" here emphasizes "comes". Ex: I was having a good day, until along came a terrible headache. Ex: He was walking down the street, then along came his ex.

What does "overdue" mean here?

When something is "overdue" it means that it is late or hasn't happened at the date it was expected to happen. Ex: Your rent has been overdue for two months. "Overdue" can also mean that something has been needed for a long time. Here, Anna is referring to the fact that she hasn't had any company or people to hangout with for a really long time, and she has needed it for a long time. Here's a few more examples of "overdue" in this context: Ex: Changes to the laws are long overdue. Ex: He feels like a salary raise is overdue.

What's the main difference between "alert", "alarm", and "emergency?"

As verbs, "alert" and "alarm" are quite different. To "alert" is to draw attention to something. To "alarm" is to cause someone to feel a sense of danger. An "emergency" is a noun, and it means a serious or dangerous situation. As nouns, "alert" and "alarm" can both mean a warning or signal for danger. Ex: There was an alert at the office for a tornado. = There was an alarm at the office for a tornado. Ex: Alert Jane that we're having an issue with the program. Ex: I was alarmed by your coughing and hoped you weren't too sick.

What does "plop down" mean?

To "plop down" means to "to sit down or lie down heavily". Ex: Stanley plopped down on the seat next to me.

Why would the speaker call a social media as a "platform"?

Social media facilitates social interaction and engagement, so they're often called platforms. Note that the meaning of the noun platform is a thing or opportunity which allows one to share their opinion or views, which is what social media sites and apps do. Ex: Social media is the preferred platform for young people to share their opinions and views. Ex: I have a presence on a few social media platforms, including Facebook, Instagram, and Twitter.