SAN FRANCISCO — If Megaupload is guilty, then who among its brethren is innocent?

On Thursday, federal authorities shut the Web site of Megaupload, a file-sharing service, and accused its operators of copyright infringement and running a vast “Mega Conspiracy.” They could face 20 years in prison.

But Megaupload was not the only such service on the Web. Many companies have crowded into the online storage market recently, most of them aimed at consumers and businesses that want convenient ways to get big data files out of their teeming in-boxes, off their devices and into the cloud — perhaps so that friends or co-workers can download them. They include MediaFire, RapidShare, YouSendIt, Dropbox and Box.net. And there are similar services from Amazon, Google and Microsoft.

All of these market themselves as legitimate ways to store content online. But they are inherently ideal for anyone looking to illegitimately upload and share copyrighted video and audio files. Most companies rarely, if ever, inspect individual files to see if the material they store on behalf of users violates copyrights, unless they are notified by someone claiming infringement.

The Megaupload indictment reminds companies that how they manage copyrighted material on their sites could determine whether they continue to operate freely or face legal consequences. At the same time, it offers a look at just how widespread such piracy is and how tricky it can be to cut down on it, given the many ways people can send files to each other online.