Yes, Venice parties at night
Many people wander around Venice after sundown, looking for some nightlife. They look for businesses that might be open, other than restaurants, where people might be socializing. They head to San Marco, and they find nothing but others looking for the same thing.
Well good news. There are two hotspots, awake late into the night.
In Dorsoduro, near Venice's University of Ca' Foscari, is Campo Santa Margherita, an unusually large trapezoidal plaza packed with "late night joints" -- restaurants, bars, gelaterias and pizza places (one of the latter is excellent, and used heavily by natives and students alike).
It's a dense, relaxed crowd of Venetians, of all ages -- along with professors, students, many foreigners but not many tourists. Order a "spritz", and watch the world go by.
The other hotspot is far more wild and dense. It's the vegetable market on the west side of the Rialto bridge. It is packed, and crazy at night.