Agree that good community doesn't necessarily equal large community. I live in Boston, which I believe is somewhere around 17%, and in the central neighborhoods I lived in, percentage of households with families was < 10%. Despite being small, it was a "good" community in that you got to know many of the other families in the neighborhood, I coached local sports, etc. It didn't feel small because of the density of the area AND people were generally on foot or transit and not driving. Running into a family we knew was a daily occurrence. All of those things contributed to making a small population feel much better.

I wonder if there are specific neighborhoods with families closer to the national average?

You say something mathematically precise and then switch into a vague "very few". Take some walks around the parks and go people watching, you will get a better feel for what that "very few" can mean.

13% of 800,000 is about 100,000. Very few.

I was restating the line you quoted, and yes precisely SF has very few families compared to other cities in the US. A sister comment seems to indicate that even NYC is 24% (I haven't verified this.) So even for similar urbanization levels SF has very few families.

Go look around. There are a lot of kids.

My point isn't about in comparison to other cities, it is more, does a smaller rate really mean "few" and how much of a problem is it really. It's not the problem that the most negative naysayers will claim. Note I didn't say something like "affordability poses no problems for families". Because it does. But there is indeed a vibrant community.