Lately, more and more people I know on Twitter are discussing the idea that it seems to have crossed, or is approaching, a threshold and has gone mainstream. What signs point to that? Celebrities, politicians and other well known non-marketing/PR types are joining in greater numbers.

For starters, a Twitter user doesn’t get any bigger than the President. President elect Obama joined the microblogging service back in March 2007, and rose up the charts to become the most followed user on Twitter with well over 100k followers. He used Twitter regularly during the campaign to communicate the campaign message of Hope and Change.

Other well known celebrities are diving into the microblogging pool.  Britney Spears joined Twitter. And then Gary Vaynerchuk criticized her for using it as another broadcast channel, not a platform to engage fans. NBA stars Deron Williams and Shaquille O’neal both joined Twitter.

In addition to people from all walks of life and business giving Twitter a try, there were 2 important book announcements this week. Twitter power users, Laura Fitton and Shel Israel announced recent deals to write Twitter books, details here and here.

The significance of these annoucements isn’t the topic, although it is an interesting one, but rather the way in which the content, stories and topics covered will be written and selected. The authors are community sourcing the writing. This gives everyone on Twitter the opportunity to submit ideas, content, tips, tools, etc… for the book.

Laura has taken it a step further, launching a Twitter for Dummies community to make this process even easier. The site allows members to:

• submit ideas
• vote on ideas
• set up your profile
• search ideas
• read tips by category
• invite friends

This is another great example of community sourcing at its finest, and one I believe we will more and more of in the future. If you’re on Twitter and have something to share, what are you waiting for? Click on over to the community and start contributing!