It's hard to tell if a celebrity is really using Twitter. When it was reported that Britney Spears had a Twitter account, popular blogger Gary Vaynerchuck, cried "Foul," accusing the star of only "kinda" posting Tweets.

Turns out for a long time, there was a fake Lance Armstrong on Twitter. He'd put out Tweets daily, saying: "Goin for a bike ride." But it wasn't him. That's no longer the case. About 10 days ago, the real cycling legend managed to take over his name.

Armstrong was the main event last night at the Web 2.0 Summit in San Francisco. He took to the stage for a fireside chat with John Battelle, Federated Media founder and host of the event. 

The seven-time Tour de France winner and founder of Livestrong.com spoke mainly about his comeback after battling cancer. There was little, if any, discussion about Livestrong, his new healthcare Web site in partnership with Demand Media, though it seemed Armstrong would have preferred to talk more about his site.

But in this segment, Battelle asked Lance if he was on Twitter.

Lance told the audience it was his 10th day on Twitter, and that he already has more followers than John Battelle, who's been a committed Twitter-er, for at least a lot longer than Lance. I'm not sure if Lance was joking, but he also told the crowd that he heard people were getting robbed on Twitter.

To be safe, if he Twittered that he was away, he'd also say that someone was at home, he said in jest. He also said he would have no interest to follow Twitter users if there wasn't a good reason to do so.

He said if people would tweet about a cause, or their music, or an organization then that would be great. If not, there would really have no point to see who's doing what and when. 

(For more coverage on Web 2.0, look below at related stories)