A key assumption in agglomeration theory is that knowledge-based firms benefit from knowledge spillovers in cities. Cities however may have different locations in the national context, such as embedded in a network of nearby cities or relatively isolated. We examine social networks employed by university spin-off firms in urban environments that contrast in such a way, namely, Delft (the Netherlands) and Trondheim (Norway). A set of growth models is explored with a focus on characteristics of social networks through which knowledge is acquired, such as tightness, strength and spatial orientation. The networks appear to differ in various respects, except for a positive influence on growth of heterogeneity in the social background of partners. The largest difference is observed in strength of relationships: an increase in strength tends to hamper growth in Delft, while it tends to enhance growth in Trondheim.