VPNs are supposed to protect you from snoops and unwanted advertising. However, four VPNs do the exact opposite: these four not only serve their own ads while running the background, but ads are also placed outside the apps, including on the home screen. The producing companies make money if you click on one of these ads.
The four VPN apps have been downloaded more than 500 million times!
You can read more in an article by Ravie Lakshmanan in the Make Use Of web site at https://tinyurl.com/privacy190922.