In response to:
And he replied “LOL. Really?” Indeed.
The chance that one of your followers has a birthday today is just 1 in 365. So the chance that they do not is 364/365.
The chance that two followers do not is the chance that one does not, and the other does not, so 364/365 x 364/365. You can write that (364/365)². The chance of three followers not having a birthday today is (364/365)³ and so on.
Since you have 2000 followers, the chance of them all not having a birthday today is (364/365)²⁰⁰⁰. The chance of that not happening is 1-(364/365)²⁰⁰⁰.
We like to see chances expressed as percentages, so we’ll multiply that by 100:
100x[1-(364/365)²⁰⁰⁰] = 99.58596373215920241300%
Homework: Compute the probability that the follower with the birthday actually read your tweet.