I feel this is something that happens when people work at startups, especially early stage ones. There is such pressure to work with your small team and put all your time and effort into this startup (like all of the people who brag about working 80 or 100 weeks --- or the OP mentioning taking a nap at the office on friday night), that you are really sacrificing opportunity to build or maintain friendships with a wider group of people. There is nothing wrong with having 3 friends and doing a lot of stuff alone, however it also doesn't have to be that way if you don't want it to be.

Do you have any old college buddies who live in SF? Email them and ask to meet up for a beer. If you are interested in dating, go on okcupid. If you like playing sports, join an intramural sports team. If you like bar trivia, just go to your local trivia night and find a team of random people to join. All of these things take time and effort, but they will result in a larger, more diverse, group of friends.

The truth is this is probably negative value for ones startup. Giving 100% of your effort to a startup will probably make it more successful than giving it 80%. But you only get one life, and perhaps more friends and a less successful startup might make somebody happier.