The United States can have a responsible and reasonable immigration policy of allowing hundreds of thousands of permanent immigrants into the country every year. We just can't continue our current policy of allowing a combined total of well over a million permanent legal & permanent unauthorized immigrants into the country each year - that kind of yearly mass immigration will lead towards a U.S. population level that could surpass one billion people by sometime in the beginning decades of the next century -- that will be too many people for a country that consumes a very disproportionate amount of the world's resources. Some of us already believe that the USA is well above a population level that is sustainable for the long-term future at the current American standard of living.
The Department of Homeland Security's most recent available statistics show 1,062,040 new legal permanent residents in 2011 and 694,193 new American Citizens in 2011. See: DHS Immigration Statistics. Nobody seems to know the precise number of net new yearly permanent unauthorized immigrants, but the Pew Hispanic Center estimated the total number of unauthorized immigrants in the USA at the beginning of 2011 to be 11.2 million. See: PEW report.
What would be a reasonable number to have as our official level of yearly permanent immigration? According to the Population Reference Bureau's most recent estimate, for 2009, the USA had approximately 328,000 emmigrants (migrants leaving the USA). See: PRB's figures. That would be a reasonable number to start at for new yearly permanent immigrants to the USA. At that level, the U.S. population will still increase due to our current demographic situation, but it might allow the country's population to be at a level near half a billion by the year 2100, rather than over 1 billion. Environmental groups in the U.S. traditionally supported an immigration level of approximately 300,000 per year, before the honorable opposition (the mass immigration movement) made it politically incorrect for American environmental organizations to take a stand on U.S. immigration.
Editor's note (4/5/12): The above "armchair" population projection includes allowing the people already in the immigration pipeline to become citizens at the current immigration rates and allowing the people in the "pathway to citizenship" to become citizens at the current immigration rates. It's for the people not already in the pipeline that the immigration and permanent resident acceptance numbers would be greatly reduced. Also, the national population increase rate is very fluid and can be decreased further, or even turned around altogether, by a combination of factors such as increased rates of emmigration from the USA, an even greater decrease in the American birthrate and, of course, a decrease in the number of immigrants and permanent residents accepted into the U.S.