It is actually being revived. During the first half of the 20th century it was used by maybe 70% of women. Then it fell out of fashion between the 70's and the 90's, when it was replaced by
Ana. Until recently, it was very unusual to be called "just"
Maria - it was seen as a "peasants" name. Now it's the poshest of trend (again, because most grandmothers and old women don't go by
Maria, but by Conceição,
Lurdes, Fátima,
Dores, and so on).
Still, its huge popularity is a little erroneous because (like
Ana) it is still very much used as the first half of a double name -
Maria Inês,
Maria Beatriz - where the girl always goes by the second name.