The aim of this study was to examine the relationship between factors of social network advertising, attitudes towards social networks, social network advertising, brands and perception of intrusiveness of social network advertising, and factors of social networks user behaviour (FOMO and privacy concerns). The research was conducted online on a sample of 212 respondents, who are digital natives aged 18 to 45. For the purpose of examining these variables the following was used in the research: a semantic differential scale taken from Elliott and Speck (1998) to measure the attitude towards advertising, an adjusted Facebook Intensity Scale (FBI scale) (Ellison et al., 2007), a questionnaire on attitudes towards brands (Bearden et al., 2011), an adjusted media intrusion scale (Li et al., 2002), the Gartner Inc. (2011) version of the social network privacy questionnaire, the Croatian version of the Fear of Missing Out Scale – FoMOs (Omajec and Čorkalo Biruški, 2020), and the Gartner Inc. (2011) version of the social media fatigue scale. The results indicate that positive attitudes towards advertising, higher levels of privacy concerns and FOMO are predictors of higher levels of social network fatigue, while attitudes towards social networks and perceptions of intrusiveness did not prove to be significant predictors of social network fatigue. The implementation of hierarchical regression analysis revealed the mediating effect of FOMO on the relationship between the attitude towards social network advertising, the attitude towards brands and social network fatigue, while the significance of mediation was verified by the Sobel test. The paper describes the findings, research limitations and offers suggestions for future research.