I've spoken about Twitter in education before as a main part of my Personal Learning Network. But I wanted to expand on it some more.
Twitter is a great social networking site that allows you only 140 characters per Tweet. This forces people to make their point succinctly, but many people use it to send out links to other articles or resources. I don't use it for keeping up with "friends" per se. I use it to connect to other educators.
Twitter is a great way to connect, share, discover, discuss, and learn about education. It's a type of on-demand professional development (which I've learned more from than years of traditional professional development).
Here are links to articles that have a list of great educational hashtags to follow. Hashtags are a way to sort Tweets by topic. You can search all of Twitter for these hashtags. As you find users who post good stuff, you can follow them directly too.
All about #edchat - a great resource on Twitter - #edchat has won awards for best Influential Tweets. This is a great place to start on Twitter.
Cybrary Man's Educational HashTags