I know there are differences of opinion, but the late Rav Chiam Pinchas Sheinberg, of blessed memory, who was the Rosh Hayeshiva of Torah Ore, held that not every tzdacha is maaser money. He said that just as in the time of the Beis Hamikdash the maaser went to the Leviiim, because they were the ones doing the Avoda, today the maasser has to go the needy bnai Torah, because they are the ones learning and as such are doing the Avodah.
He was very makpid on this, and even when he was building Torah Ore, many years ago, and it was so important to him to have the building finished already, he never used masser money for the actual building. If someone told him that they were donating maaser money, he used that money for the food for the bachorim, but not for the building.
