Facebook allows for 3 options:
Twitter will only allow for accounts to be deleted after supplying very detailed and thorough information. You will be required to provide the following information about the deceased via mail: Twitter account's username, a copy of the deceased user's death certificate, a copy of a government issued ID of the person requesting deactivation of the account, a signed notarized statement including the first & last name of the person making the request, current contact info for the person making the request to include an email address and the status of the relationship between the requestor and the deceased, a written description of the request, a written description of the evidence that the account belongs to the deceased if the name on the account does not match the name on the death certificate and a copy of the obituary (online or print). Mail all items to: Twitter Inc., c/o: Trust & Safety, 1355 Market Street, Suite 900, San Francisco CA 94130. Contact can be made via e-mail at privacy@twitter.com.
Instagram allows for options similar to Facebook and requires the full name, the username, a copy of the death certificate and a copy of the obituary (online or print) to deactivate an account. Visit help.instagram.com and select the "Privacy & Safety Center" link. Choose "Report Something" and then choose "Other Types of Reports". Review the information provided to delete or memorialize an account.