Twitter 5th Birthday: Lady Gaga, Justin Bieber, Barack Obama Most Followed

@Twitter: #Happybirthday! 5 Yrs Old!

Twitter's San Francisco staff has not done much publicly to mark today; they had more of a celebration in March, the five-year mark from when they put the site online for so-called beta testing. That kind of thing is common with tech startups; Google says it celebrates its anniversary on different days in September, "depending on when people feel like having cake."

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What was on people's minds today? It's a summer Friday, and some of the topics Twitter listed as trending showed a pattern: Neville Longbottom ... Alan Rickman ... Gryffindor -- you get the idea. People were commenting on the new Harry Potter film.

So there we are, tweeting on the serious and the salacious, the weighty and the light-hearted, the trivial and the truly important. It's where Jim Carrey, the actor, posted a picture of his new grandson: "This is what 7lbs 11 oz. of Calfornia [sic] dynomite [sic] looks like!"

ABC News' Brian Canova contributed to this story.