Many publications estimated that as many as 53 million songs from 14 million artists were affected by the data loss, but it wasn’t clear how much of that music was uploaded by users. (When Myspace rebooted in 2013, it boasted a library of 52 million songs thanks to deals with labels and uploads from users, according to reports at the time.)

The news was the latest chapter in the long decline of the once-mighty social media giant. Founded in 2003, a year before Facebook, MySpace boasted about 250 million users in the United States in its heyday. In 2005, Rupert Murdoch’s News Corporation paid $580 million to buy the site’s parent company. Around that time, Myspace.com also became the most-visited website in the United States, briefly overtaking Google. But it changed hands two more times in the last decade for a fraction of that price, as Facebook, Reddit, Twitter and other platforms lured users away.

Myspace grew to be a formidable force in music hosting, at one point amassing the biggest library in digital music. But it struggled on that front, too, eventually losing ground to other services like Spotify.

For those who kept their accounts, the news of the data loss comes as little surprise. They have complained in Reddit discussions and elsewhere about receiving similar messages from the company for more than a year. Over the weekend, the frustration spilled into sight again after a much-cited tweet by Andy Baio, a former chief technology officer of Kickstarter.

Jordan Tallent is among the MySpace users who has been trying to recover his music. Last summer, Mr. Tallent, a 26-year-old audio-video professional in London, reached out to Myspace to ask for help recovering songs that his former post-hardcore/heavy metal band, Where Got Ghost, had uploaded to Myspace.