I disagree. Three of my best friends were former co-workers. We happened to be new-ish to the city back then, in our 20s, going out, bars/clubs, etc, yet sharin

I disagree. Three of my best friends were former co-workers. We happened to be new-ish to the city back then, in our 20s, going out, bars/clubs, etc, yet sharing a lot of other common career goals as well as personal ones. If you hang out with co-workers on non work related (or sponsored activities), a lot, there is a greater chance for you to become friends on the long term after you moved on to different jobs. They key is to have other common interests that are not related to work, and hang outside work related activities.