According to a 2013 article in the Global Times there were 200,000 foreigners lived in Beijing, up from 180,000 in 2011. The average stay of these people, at least those that work, is 5 years. 

Foreigners come for a variety of reasons. Many work for national governments. Many come to work in global companies who are headquartered in the city. They are part of the managerial class, and have specific parts of the city they inhabit--the Chaoyang district and nice apartments around Sanlitun or Guomao, or in larger houses (little simulacra Americas or Europes) out in suburbs such as Shunyi District by the airport. Though as China's managerial class there is less need for this level of ex-pat.

Next are students. There are many students from around the world in Beijing. Some are studying Chinese for only a short time. Some will stay for graduate degrees and China has relationships with a number of countries. I taught classes of international students (mainly South Korean, but also Indonesian, Burmese, Thai, Kazakh, Haiti, Iran, & Mongolian) who were getting 4-year Bachelor's degrees at Tsinghua. This is a trend as China tries to build its Universities world rankings by recruiting global students. 

Then there are people passing through. Working as English teachers or writers or artists, drunks, entrepreneurs, and backpackers who never left, looking for adventure or washed up with no where else to go! China has always attracted a wide range of these types, people who find life here easier or more colorful, for many reasons. 

There is a vibrant culture of foreign life in Beijing. Different worlds that sometimes interact with the China around them, and sometimes not. It is always interesting to analyze how outside of one's comfort one those go in a new context. There are places in Beijing where you can replicate the life you lived at home, perhaps with a higher standard of living for comparable work and cheaper beer. . . this is no doubt part of the appeal. But there are other places where the two world, or multiple worlds, mix in interesting ways and create new dimensions of the city's culture. 

Or just watch this video. . . Laowai Style