I'm relatively young (17), so I can offer some perspective from the teenage side. I definitely think this is one of those issues where it is going to vary greatly with age.

This article is very accurate, at least with regards to my experience. Ending a text with a period generally indicates annoyance or frustration with whatever the previous text was (and can sometimes indicate anger, too). It's really a passive aggressive way to do so, however. If I received "Ok." in response to a suggestion or statement that I had just texted, I would immediately assumed that the other person is upset in some way. I would follow up with something like "What? We could do something else if you want" or something similar.

"Let's stay in" sounds like a sincere suggestion to me; if I were to disagree I would not hesitate to say it and I would assume you to be open to other suggestions for what to do that night. "Let's stay in." is much more firm, almost like a command. I would expect a lot of resistance to doing something other than staying in, and wouldn't bother suggesting something else unless I was really opposed to the idea of staying in.

Exclamation points indicate either sincerity or extreme sarcasm, entirely dependent on context. The question mark still retains its role of forming a question, but, as the article stated, it can make certain statements seem less cocky. It is almost like casting doubt on whatever you just said. "I think I got the highest grade?" Maybe I did, but I am acknowledging that it is possible or even likely I did not.

This is all really interesting stuff. It's fascinating how certain punctuation has picked up these connotations. I've never thought about it before, yet it is completely true.