In a 2019 interview John Anderson interviewed Konstantin Kisin (excerpt, original video). The discussion is transcribed by the OP of Were over 3000 persons arrested in Britain for social media posts in 2018?. I is the interviewer and K is for Konstantin.
K: In Russia last year 400 people were arrested for things that they posted on social media. Obviously this country is very different. How many do you think were arrested in Britain for what they said in social media?
K: Take a guess.
I: I've no idea.
K: 3300 I: Really? Arrested for things that they said on social media? ...
The 3300 figure is most likely taken from a 2017 piece in The Times (thanks to this answer).
More than 3,300 people were detained and questioned last year over so-called trolling on social media and other online forums, a rise of nearly 50 per cent in two years, according to figures obtained by The Times.
But the statement is phrased such that the numbers certainly need not be restricted to the offences cited by the times.
How many people were arrested in Russia in 2018 for social media posts, and what definition of "for" is this figure based on?
Related question: Were over 3000 persons arrested in Britain for social media posts in 2018?