Here is a long paper with a lot of references on social network theory and empirical evidence, including learning and diffusion of innovations. For example,

A nice example of this using field data is a study of social learning by Conley and Udry (2001, 2004). They examine the use of fertilizer by pineapple farmers. In particular, they show that changes in the amount of fertilizer used by a given farmer are related to the success or failure of similar past changes in fertilizer use by other farmers. Having controlled experiments can substantially narrow down the range of explanations for observed peer correlations. For example, Hesselius, Johansson, and Nilsson (2009) examine absences in the workplace based on a randomized rule affecting about 3000 workplaces in G oteborg Sweden. Randomly
assigned agents were allowed to have longer spells of absence from work (14 days) without having to produce a doctor’s certificate than was the rule for the general population (8 days). This resulted not only in an increase in absences for the treated individuals (those allowed the extra time before producing a doctor’s certificate), but also for non-treated individuals conditional on being in a workplace with many treated individuals. Interestingly, the affect of how many other treated individuals there were in the workplace did not significantly influence treated individuals’ behavior. This allows them to distinguish between various ways in which the peer effects might work, ruling out things like enjoying time together and being more consistent with a fairness effect or related peer effect on preferences. This sort of study shows the power of (field) experiments in identifying peer effects…
The list of settings where peer effects, or network effects more generally, have been found to be important is a long and varied one. It includes a range of things from criminal behavior (Reiss (1980), Glaeser, Sacerdote and Scheinkman (1996), Kling, Ludwig and Katz (2005), Patacchini and Zenou (2008)), to education (e.g., Calvo-Armengol, Patacchini and Zenou (2008)), to risk-sharing and loan behavior (Fafchamps and Lund (2003), De Weerdt (2004), Karlan, Mobius, Rosenblatt, Szeidl (2009)), to obesity (Christakis and Fowler (2008), Fowler and Christakis (2008), and Halladay and Kwak (2009)). (See Fafchamps (This volume), Ioannides (This volume), Jackson and Yariv (This volume), Munshi (This volume), Sacerdote (This volume), and Topa (This volume), for more examples and background on empirical evidence.)