We study the relationship between two distinct dimensions of social capital (bridging and bonding social capital) and the personal performances of individuals: their reported subjective well-being (SWB) and earnings. A theoretical model is put forward which explains the sources and dynamics of social capital formation. It predicts an inverse U-shaped relationship between any type of social capital and SWB, an inverse U-shaped relationship between bridging social capital and earnings, and an unambiguously negative impact of bonding social capital on earnings. The key predictions of the model are confirmed using cross-section survey data from the 2005 wave of the “Social Diagnosis” survey program conducted in Poland. Very low levels of bridging social capital observed in Poland imply that it is unambiguously beneficial to invest in it: both SWB of individuals and their earnings would increase in such case.