How does Politeness work in Social Media?

Questions to consider:

How does politeness interact with social media, blogs, vlogs and minimal space mediums like tweeting?

Isn't politeness the grease of the social clockwork?s this grease ignored entirely for reasons of shock/attention span or character space? Or did it evolve and change forms?

We know some of the evolution:
All caps = yelling
Terminal periods on texts and tweets seem severe, nay even aggressive. Crair says that line breaks are more effective than the humble period, and terminal periods are completely unnecessary and therefore if they are used, they hold a different meaning, that being one of "pissed."

What about things like  :), or ;p, Where do they come in for politeness? When and how are they properly used?

When does discussion get uncomfortable? or offensive?

Does proper sentence structure show respect to your topic?