Has anyone else noticed this form of sentence structure?

Yes, it is an age thing and/or cultural difference.

The young people (I don't consider myself among their numbers, anymore) have adopted a rather common bit of conversational convention. Conversationally, the "so" or "okay" is used as a means of transition, signaling a shift in conversational tone: e.g. "That's really great. So... (implicit change of topic to more serious fare), I've been thinking about getting a divorce."

It's completely arbitrary, as far as I can tell, that "so" is among the more common transitional words. It doesn't, in this context, suggest familiarity. Instead, it signals the speaker's humble request for help when facing the unpleasant circumstance of his or her own ignorance about something.

I don't think they are worrying about their "sentence structure". People are just writing questions in a conversational way, as we would in common speech. Well, what do you think of this? So, what's that all about?

I am surprised you have never heard this usage before, it's very common in English. Using words like well, so and OK as exclamations has been around for a long time, particularly in spoken English. It doesn't always mean that the writer is assuming you are familiar with their situation, but since they are writing the questions within the context of a topic, such as Words and Word Play, they are perhaps assuming you have read some of the preceding questions.

"So,..." implies the speaker's

1) desire in changing the topic or introduction of a new topic, and

2) checking if he/she understands what his/her opposite said.