The author probably could have argued their point better, but the point is that if you spend 600 hours on social media on average per year, you could take away 2/3 of the time and read 200 books (or something else). Your argument that reading for the sake of reading is valid, but I would argue that most social media is trash (in varying degrees -- do you need to know what your friend or some celebrity was thinking at the time on Twitter, or see the 50th friend wish everyone a great 2018 on Facebook?) and the books that a conscious reader would choose would be less like trash. Also, it isn't that you should read for the sake of reading, it's that you could spend your time doing something probably more mentally healthy or a better use of your time than social media by reading.