The whole sentence is rather odd, to me. "When clicking" is a strange way to word that action, and the future continuous sounds out-of-place as well. Sounds like video game instructions. I would simply say, "When you click the Follow Button, you will follow your target."

My guess would be that the website designer made a mistake or did not fully understand English grammar. The correct way to make that sentence read correctly would be "After clicking the button, the user will instantly be following the target user." This would keep consistency with the future reference. The continuing future reference would be correct in this instance, because the user would not cease to follow the target user until some user action caused the following to stop.

Future event, continuous action. :)