I'm not a huge fan of Memrise, especially for Chinese. At least when I checked it, there was way too much of an initial emphasis on characters and it also suggested a widely-believed but incorrect idea that characters = words. I say this as someone who has spent most his adult life in the Chinese speaking world, gone through the struggle of learning myself and met hundreds of other foreigners doing the same.

The best option for learning Mandarin would be Popup Chinese. If you exhaust their materials or find yourself wanting a southern accent, go on to ChinesePod. Once you can already speak and read some, go to Skritter if you want to learn how to write. If I had had these three resources 10 years ago, it would have been a much, much easier path.