If I google your question MAIN PARTS OF SOCIAL MEDIA MARKETING then (after the advertising), I get hits for 3, 4, 5, 8 , 9 or 12 essential elements on the first result page. So it seems there is no unique answer.
Better read all, then decide how many and what are essential for *your* business.
Numerous factors are crucial for social media marketing, which is a broad topic. Some of these factors include establishing an audience identity. Budgeting. Goal-setting and objectives. Team building/resourcing. Do some research. selecting your platforms. generating content. incorporating it into your whole digital strategy.
Social media marketing is a loose concept, from spamming links on various sites to targeted advertising in search of a target audience. If you take instagram, it is just targeting advertising, building quality content, enticing users. Many pages on instagram use auto-following, i.e. live people subscribe to the page and start to write comments, give likes, as a result the page becomes active and subscribers grow. When the page is already fully well-established marketing that steadily attracts new people to the page, it already helps to bring profit if you do it right.