[ovb]

The divisions used in the article are Metropolitan Statistical Areas (MSAs), which are defined by the Census Bureau. According to the standards they use for delineating MSAs (metropolitan areas containing a core city of at least 50k inhabitants) and µSAs (metro areas with a core city of between 10k and 50k inhabitants): http://www.whitehouse.gov/sites/default/files/omb/assets/fed...

Two adjacent CBSAs will merge to form one CBSA if the central county or counties (as a group) of one CBSA qualify as outlying to the central county or counties (as a group) of the other CBSA using the measures and thresholds stated in 3(a) and 3(b) above.

[CBSA is the umbrella term for MSAs and µSAs]

3(a) and 3(b) are:

A county qualifies as an outlying county of a CBSA if it meets the following commuting requirements: (a) At least 25 percent of the workers living in the county work in the central county or counties of the CBSA; or (b) At least 25 percent of the employment in the county is accounted for by workers who reside in the central county or counties of the CBSA.

So it sounds like if 25% of San Francisco County residents work in Santa Clara County, or 25% of SF jobs are taken by Santa Clara County residents, they would merge the two MSAs. Presumably this is not the case, and thus the MSAs are separate.

The Census does have a higher-order concept called a "Combined Statistical Area". The whole Bay Area is lumped into one CSA: http://en.wikipedia.org/wiki/San_Jose-San_Francisco-Oakland,....