Before today, I knew: the reason Twitter was originally limited to 140 characters was so it could be transmitted over SMS (which was limited to 1120 bits, or 160 7-bit characters).
TIL: the reason SMS is limited to 160 7-bit characters is because Friedhelm Hillebrand, who was the chairman of the non-voice services committee within the Global System for Mobile Communications (GSM), found that a) the average sentence was <160 characters, and b) that matched the average postcard.