Twitter is a fun playground for many authors, agents and editors. They're a very open and giving community, willing to participate in designated Twitter chats where you can ask questions about the industry, trends and anything else that your heart desires relating to the topic.
This fab group also respond to individual tweets in you may have sent them - as long as it's not a creepy tweet or an unsolicited pitch.
It's not cool to pitch via Twitter to an agent or an author unless they are holding a specific contest that requires it.
Getting on well with an agent/author on Twitter will not guarantee you'll get a publishing deal.
Don't rant on Twitter bagging out an agent/editor for rejecting your work. It'll get back to them and score you a bad rep - hello google alerts!
Remember that the writing industry is subjective. A rejection doesn't mean that your work is not good, it just means that they didn't feel strongly enough about the project you're querying on to take you on (a friend of mine is getting published and scored her agent on around the fourth manuscript she'd queried her with - don't give up, keep writing).
You can follow these guys on Twitter, but it doesn't mean they want to friend you on Facebook.
This is all great advice. I remember having to dissuade a friend of mine from following all of the agents that she wanted to query and striking up conversations with them -- reminded her that just because they might recognize your name doesn't really give you a push in the right direction.