According to the government, the total population of Singapore is 5.26M. Out of this, 3.27M are citizens, with 540K Permanent Residents and 1.46M non-residents. In other words, about 2M or 38% of the population were not born in Singapore.[1] Note that this is a conservative estimate since an immigrant can become a Singapore Citizen after 2 years as a Permanent Resident.
And yes, obviously there is great concern because a lot of foreigners do not assimilate well either.[2]
[1] http://population.sg/resources/population-composition/ [2] http://www.migrationpolicy.org/article/rapid-growth-singapor...