To report offensive posts or Cyberbullying accounts on
- To report someone’s account, go to their profile and click next to their username. Select Report user. …
- To report a post, click below the post. Then click Report inappropriate and follow the on-screen instructions.
For more information see: https://help.instagram.com/519598734752872
Facebook offers it’s users detailed information depending upon what you need to report.
Twitter has a trust and safety section that explains in very easy terms what to do (I advocate muting and reporting but the choice is up to you):
Tumblr has its abuse page here: https://www.tumblr.com/abuse Make sure you report under the correct category.
WHEN REPORTING ON ANY SOCIAL MEDIA PLATFORM:
Remember to use the advice here. To make sure you have gathered proof that you can recall later or use in your reports to the social media platform.