Facebook Inc., Alphabet Inc.’s Google and Twitter Inc. are set to divulge new details showing that the scope of Russian-backed manipulation on their platforms before and after the U.S. presidential election was far greater than previously disclosed, reaching an estimated 126 million people on Facebook alone, according to people familiar with the matter, prepared copies of their testimonies and a company statement.

Facebook estimates that 470 Russian-backed accounts connected to a single pro-Kremlin firm, the Internet Research...