Actor Robert Downey Jr. has joined Twitter and managed to get almost one million followers in less than one day.

He opened his Twitter account on April 11. The "Iron Man" star posted his first tweet on April 11 afternoon and amassed almost one million followers in less than 24 hours, reports femalefirst.co.uk.

The 49-year-old made his debut on the social networking site by sharing a photograph of himself wearing sunglasses and holding a sign bearing his new verified Twitter handle @RobertDowneyJr, captioned: "Talk to me, Twitter".

After encouraging his fans to get in touch, he admitted he was still struggling to get to grips with the platform, writing: "Loving all the love, folks. It's been a blast. Though can somebody please explain how anyone can keep their thoughts to 140 characters or le (sic)."

His short bio states: "You know who I am."

Some of his other well-known followers include Israeli supermodel Bar Refaeli, British actor Ian McKellen and chat show host-comedian Ellen DeGeneres.