This is the 605th most frequent Finnish word.
sosiaalisen
"Sosiaalisen" means "of social" or "social" in the possessive/genitive form.
Here, 'sosiaalisen' is part of the term 'sosiaalisen median', which translates to 'social media'. The word 'sosiaalisen' is the genitive singular form of the adjective 'sosiaalinen', agreeing with 'median'.
Hän on erittäin aktiivinen sosiaalisen median käyttäjä.
He is a very active user of social media.
Here, 'sosiaaliseen' is the illative singular form of 'sosiaalinen', agreeing with 'politiikkaan' (policy), indicating direction or involvement.
Hänen työnsä liittyy sosiaaliseen politiikkaan.
His work is related to social policy.
In this sentence, 'sosiaalisen' is used as a genitive form of the adjective 'sosiaalinen' to modify the noun 'median', which translates to 'media' in English. This forms the expression 'sosiaalisen median', translating to 'social media'.