  In the last few years, Online Social Networks (OSNs) attracted the interest
of a large number of researchers, thanks to their central role in the society.
Through the analysis of OSNs, many social phenomena have been studied, such as
the viral diffusion of information amongst people. What is still unclear is the
relation between micro-level structural properties of OSNs (i.e. the properties
of the personal networks of the users, also known as ego networks) and the
emergence of such phenomena. A better knowledge of this relation could be
essential for the creation of services for the Future Internet, such as highly
personalised advertisements fitted on users' needs and characteristics. In this
paper, we contribute to bridge this gap by analysing the ego networks of a
large sample of Facebook and Twitter users. Our results indicate that
micro-level structural properties of OSNs are interestingly similar to those
found in social networks formed offline. In particular, online ego networks
show the same structure found offline, with social contacts arranged in layers
with compatible size and composition. From the analysis of Twitter ego
networks, we have been able to find a direct impact of tie strength and ego
network circles on the diffusion of information in the network. Specifically,
there is a high correlation between the frequency of direct contact between
users and her friends in Twitter (a proxy for tie strength), and the frequency
of retweets made by the users from tweets generated by their friends. We
analysed the correlation for each ego network layer identified in Twitter,
discovering their role in the diffusion of information.
