Not the ADA specifically, but American courts have generally held that large commercial websites do need to be accessible in terms of the ADA.
That's aside from the cruelty of diminishing accessibility on a major social platform, slowly cutting off disabled users, despite its cost likely being a rounding error in the greater scheme of things.
That's aside from the cruelty of diminishing accessibility on a major social platform, slowly cutting off disabled users, despite its cost likely being a rounding error in the greater scheme of things.