In good-sounding idiomatic usage,I would use:
users' social networks and not: social networks of users.
A popular source of information [singular subject] is [therefore a singular verb] is users' social networks.
Users' social networks [plural subject] are [therefore plural verb] a popular source of information.
There is no rule about using the possessive but it does sound better to the native speaker's ear. Whereas "a popular source of information" sounds as good as here the equally correct "a popular information source".
See academic usage:
Influence of Topical Interests on Users’ Social Networks
In this study, based on a social conference support system Conference
Navigator 3 (CN3), we explored three kinds of knowledge sources to
generate recommendations: users' preference about talks (CN3
bookmarks), users' social networks (research collaboration network and
CN3 following network) and talk content information (titles and
users' social networks
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Please note: there is also quite a bit of usage such as:
cocaine users' social networks or drug users' social networks or adults' social networks or teenagers' social networks.