Here's the thing.... Getting online isn't about avoiding people; it's about embracing a new way of engaging others. I communicate online, yes - but I've also met a lot of people online who have turned into truly good friends, dates, business contacts, etc. Face it - the world changes. We can be ready to embrace new ways of participating in life - or prepare to be left out of the loop.
While I'm all for "new" in terms of how and when we communicate, I also believe that some traditional rules should transcend from in-person to online communication.
Etiquette is any conventional requirement pertaining to social behavior. Observing etiquette indicates that one is observing proper conduct as established by, or for, a community or occasion.
Why should the online community be void of the same requirements (courtesy, respect, etc.) as the in person, or conventional, community? It's still a community, a group of people coming together to share.
At a party, would you turn your back to one person, excluding him from the conversation? No. So why would you @reply to only one person in a twitter thread, when the conversation clearly includes others?
At a family reunion, would you start screaming at a relative, or share your most intimate personal secrets in front of everyone? No. So, don't do it on their facebook wall, where anyone and everyone can read.
When you speak to your coworkers, do you use proper English? Yes. So when you send an email, don't ignore grammar or spelling. Taking the time to "say" something correctly is a sign of respect and it's a better reflection of who you are - just like speaking properly is when you're in person.
I could go on, but you get the idea. I'm simply saying that respect is key in any communication. No one is perfect, but it's important to do your best.