It seems to me like there have always been very few successful diets. They seem to become trends more than anything else, followed by millions until rooted out by scientists and nutritionists as actually being incredibly unhealthy and possibly damaging.
The Atkins diet is perhaps the best example of this. At one point there was said to have been 1 in 10 people in the USA following it. This healthy diet plan focuses heavily on not eating carbohydrates and holding off from eating protein until you can't bear it any longer. Eating mountains of fruit and veg is supposedly a balanced diet. There was a lot of controversy over the diet about the extremities Atkins encouraged and the fact that carbs are needed, at least in moderation, in a balanced diet.
Another example of a fad diet could be said to be the Master Cleanse diet. This literally bans all food and replaces it with tea and lemonade made with maple syrup and cayenne pepper. It was promoted as a detox diet but possibly brought up more controversies than the Atkins as followers were almost guaranteed to put weight back on after the diet, there are short term side effects such as nausea and fatigue and long term effects include loss of muscle mass.
It is difficult to know how long it takes for diets to be found out as doing more harm than good. The Dukan 100 foods diet has just celebrated its second year online birthday and looks to be gaining more followers still. The plan focuses on telling you what you can eat rather than what you cannot, with personalised information regarding how best for you to proceed at all times. There are hundreds of recipes to try meaning it is difficult to get bored of the foods.
When you first sign up, you are given an overview of what to expect from the diet and how you will be helped to get to the weight you want to reach. The four Dukan diet phases (Attack, Cruise, Consolidation and Stabilisation) are all explained on the website and before you pay a penny you are shown how long you would potentially spend on each.