Spanish is, without a doubt, the main "second language" of the United States. Go to any city of reasonable size, and you'll find signs, advertisements, and print media all available in Spanish. I live in Chicago, and a good third of the billboards are in Spanish. Teenagers often pepper their English with Spanish words and phrases, and similar practices are prevalent in pop music. In addition, Hispanics and Latinos are far and away the largest minority in the US, comprising 17% of the population according to the latest census, and you will find towns along the Mexican border where Spanish is the primary language. Add to that the very real possibility of Puerto Rican statehood in the coming decades, and Spanish appears to be firmly entrenched.
However, this all comes with a caveat: virtually all people in the US who use Spanish as their primary language are immigrants. Retention of the language by later generations is quite low, as is the case with most immigrant populations throughout modern history. The current influx of Spanish-speaking immigrants, while considerable, can't and won't last forever, especially as Latin American countries like Mexico and Colombia gain their economic footing - much of Latin America now offers the same or similar standard of living that drew countless immigrants northward. Even in Puerto Rico, English is making strides among the younger population, though it has a long way to go before it matches Spanish in strength on that particular island.
So my question is: what will happen to Spanish once the immigration wave is over? Will it stay strong and become the US's secondary language indefinitely, or will it fade out with its speakers?
By the way, there should be a preposition somewhere in that title.