Israel was admitted at the UN within the 1949 armistice line, not within the 1947 partition plan. The 1947 partition plan was rendered null and void after the Arabs rejected Resolution 181, which was not even binding anyway. It was merely a recommendation. Initially, the Jews accepted it because they sought international legitimacy. But once it was rejected by the Arabs who violated it, there was no longer any reason for the Jews to stick to it.
Police disperse hundreds in brawl in Kafr Manda, northern Israel (Haaretz)
from the article: An open letter to Mahmoud Abbas: Save the two-state solution