Found a lost wallet on the street with a lot of cash

I understand your concern regarding your social media account and I would like to offer you some advice as per my knowledge that might help you out. 
Since there is someone who has unauthorized access to your social media, firstly I would like to ask you to please change all your passwords as soon as possible and then rush to the nearest police station in your area to file a complaint under the provisions of Information Technology Act, of 2000.
You can refer to sections 43 and 66 of the said act as these sections impose penalties on the wrongdoers, criminalize hacking, and impose penalties and punishments for the one who gained unauthorized access to someone’s account, respectively. 
Filing a complaint under these sections will protect you from such security breaches and the team will conduct thorough research and punish the criminal. 
Cyber offenses are also addressed under the IPC. Section 66C of the IT Act and Section 499 of the IPC tend to tell us about theft and defamation respectively and both of these concepts can be applied in your case as you have mentioned that the hacker has also sent some inappropriate messages in your name. 
If you feel that the messages harmed your reputation, then you can go for the defamation provisions. 
In the case of Shreya Singhal v. Union of India (2015), the Supreme Court addressed the need for encouragement for victims suffering cybercrime and online privacy violations. So according to this, you can also move to the Cyber Crime Cell by providing all the evidence. 
I also suggest you alert your contacts regarding this to avoid any misunderstandings.

Admittedly, you have done a wrong. Onus rests with you in initiating the remedial action.

It is as good as stealing the property belonging to others.

You were required to hand it over to police, but by retaining the same you are committing an offence.