The 72-year-old actor took to micro-blogging website Twitter, on which he has over 15 million, to share the news with his fans.
He wrote: "T 1900 - FaceBook 21 MILLION...!! Thank you all.. aiming for 30 MILLION now!! and then more..!! YEAAAHHH!! BAAADOOOMBA!!"
Amitabh's Facebook page on Wednesday had 21,008,304 likes.
Meanwhile, on the professional front, the 'Shahenshah of Bollywood' is prepping for "Wazir", which also stars Farhan Akhtar.