I think online typing says A LOT about a person. Today's generation drives me crazy with all the abbreviations and acronyms. Unfortunately, I think texting and social media is to blame for a lot of this.
A made-up example of what I'm talking about: I wuz wonderin if U R goin to the prty & if U R can I gt a ride w u??
Now take the growing generation. They get cell phones, IPAD's, and lap tops by age four and this abbreviated language is what they are surrounded by. When they get to first grade they literally think that spelling "laughing out loud" is spelled LOL.
The way people type shows their intelligence level also. They might be grammatically correct but the content shows that they do not have a clue what they are talking about. The opposite is true also. Someone may be very knowledgeable about something but have a paragraph that is one run-on sentence. Some people are just lazy. They write down their thoughts and then neglect to proof read what they wrote.
The words people use and the context of what they are writing tells about the person. For example, if I was writing a blog about being an ice hockey goalie, then chances are I like ice hockey, especially the goalie position. If in that blog I used a word like "awesome", it might indicate what generation I grew up in.
Regardless of what the reasoning is, a personality comes out in a person's typing, word usage, and grammar.