Here it comes. The tipping point of Twitter.In the past week, two independent friends have told me they are signing up to Twitter. Both of them aren't tech or social media geeks. However, they are both extremely popular, very influential and both have a large following on Facebook.This was before Rove spoke about it on Sunday night.Twitter is about to go mainstream. Although when it does, I think you'll find its design flaw will cripple the social network....