Seattle is the fastest-growing big city in the country, its population just passed 700,000, and more than 1,000 new people are moving here every week.

But where did they come from? LinkedIn is on the case, you’ll never guess what it found. Oh wait, yes you will, it’s San Francisco.

As part of its monthlyWorkforce Report, LinkedIn looked at hiring and employment trends in 20 major U.S. cities, including Seattle. The study found that for every 10,000 people with a LinkedIn account in the Seattle area, approximately nine people came from the San Francisco Bay Area some time in the last year. The other most frequent sources of migration are the nation’s largest cities, New York, Chicago and Los Angeles.

Believe it or not, some people actually leave Seattle as well. The most popular destination for people skipping town: Boise.

Overall, the study found that Seattle has the most gross migration, the number of people who come here plus the people who leave, with San Francisco. That is followed by Los Angeles, Portland and New York. A decent number of people made the trek across the Cascades, one way or the other, as Spokane, Wash. is another frequent source of migration in and out of Seattle.

That San Francisco has the most migration back and forth between Seattle is no surprise. More than 90 companies have opened up engineering centers in the area, many from the Bay Area. Some out of town companies, like Facebook and Google, have grown so big, so fast that they are among the top tech employers in the city.

We’ve studied these trends in the past and found that Bay Area residents come here not just for the employment opportunities, but for the quality of life as well as the comparatively (for now) affordable housing market. The interaction has become so well known that The Economist profiled the trend earlier this month in an article titled Silicon Valley North.

Fitting this trend, the skillsets with the greatest oversupply in Seattle are almost entirely tech related — things like cloud computing, proficiency in various programming languages and software development. The scarcest skills in Seattle, in which employers don’t have as many qualified people to choose from include: healthcare management, teaching, sales, accounting and finance.