Google's controversial email scanning practices netted a child abuser last month, but the internet giant is not the only technology company proactively combating the sharing of child abuse images.

While it is not known exactly what tipped the Google algorithms off to the existence of three pornographic child exploitation images in John Skillern's Gmail, the registered sex offender is now facing new charges in the US.

Analysis service: PhotoDNA software watches for child exploitation images online. Credit:Bloomberg

Both Facebook and Twitter use a photo analysis service called PhotoDNA from Microsoft. The software, launched in 2009, is also used on Microsoft's email service Hotmail (now Outlook), search engine Bing and data storage and sharing service SkyDrive.

The image-matching technology attaches a unique number (or hashing string) to images identified by child abuse prevention groups and recognises matches when uploaded and shared by users, regardless of whether they have been modified or resized.