The net is far too low to be a volleyball net. Based on the paddle in her hand, I would imagine it's some kind of paddle/racquet game, presumably one that is popular in Brazil.

EDIT: To elaborate: the top of the net in women's volleyball is 2.24 m (7' 4") above the ground. In order to be head and shoulders over it Miku would have to be pushing 2.4 m / 8 ft tall. It's more likely to be a Beach Tennis net, which is a much more reasonable 1.7 m / 5' 7" high.