I used iknow when it was called smart.fm and i studied 4033 words and it told me I mastered 2270 words. over about a year I accumulated 36 hours and 57 minutes of time spent on this service. I really did like it however today you have to pay to use the service. currently the statistics and metadata on your studying seems really awesome although a lot of that I did not get to use on smart.fm
What makes this better than anki is it gives you multiple choice questions and quizes that you type in your answer.
Memrise is pretty new and is my favorite, it feels more organic than iknow. there is an option to "plant" new seeds and in a planting session you plant 5 seeds , It shows you the word , gives multiple choice , then you type it in. this is the only time you see the same word in the same session, vs iknow where you can keep getting the same word in the session, it feels like you can progress faster and learn more words. you still get the spaced based advantage if you missed a word but it's just not annoying like smart.fm can be. One more reason why this is my favorite flashcard service is you get points and you can still get some credit or points if you misspelled or got close to your target answer. There are also volunteer curators of the dictionary's that can fix errors in user contributed lists, and memrise also tries to limit re-learning the same word in different lists. Another reason that really makes memrise superior is it's use of mem's these are comments/pictures or anything that can help you remember an item. anyone can add them and you can choose your favorite if there are multiple mems for an item, it shows you them when you are planting words and when you made a mistake on a word. This makes memrise VERY powerfull. I recommend everybody should try it.
Supermemo is very similar to anki where you choose *if* you got it right. and there are lists that are free and lists that you pay for. I used it a bit before i used smart.fm and it's my least favorite of the 4