Okay, I had never heard of memrise. I have used similar "flash card" or SRS software before. And you can see that memrise is only one many on the market.
I decided to give memrise a try, using especially to "memorize" or learn some Middle English vocabulary, since I am reading Chaucer these days. As in most these programs, one can create one's own "flash cards." I made certain to create some that used example sentences for each tested word. Because I am firm believer that words are best learned and retained if used in a sentence. This goes even further, when I then come across a tested word in a text that I read in my free time, so then I get the benefit of seeing the word in two meaningful sentences; and as far as I am concerned, that locks it up into my long term memory. And since I am continuing to read Chaucer and other authors who write in Middle English, then I expect to see these words repeated again (and again).
To my surprise, there are actually a handful of existing memrise "courses" designed to "teach" Middle English vocabulary (and also some in Old English). Some have sample sentences as part of their design. Most do not. You can guess which ones I appreciate and learn from the best. But even one particular course, which is "straight" learning of new words with no sample sentences: I noticed that words in memrise that I had encountered recently in my own reading were much easier to learn using memrise. And I'm sure they will stick in my memory longer, because of my encounter with them in my recent reading.
As I said, I think it can be used for learning words for a set task, like a vocabulary test or the GRE. But if one wants to use it for much more than that, one absolutely must incorporate some kind of context (example sentences, preferably **meaningful* example sentences, which would include those read from a text) into the design of one's memrise flashcards (or the flashcards of whatever SRS software one is using). I just call them flashcards, because these programs incorporate the equivalent of electronic flashcards.
An excellent article, How to Remember Words When Learning a Language, I highly recommend. It stresses the need for context, and it gives four methods of learning vocabulary, only one of which is SRS programs. Another one sounds similar to what rogermue is discussing. Also, in the comments to the article there are some insightful things, such as the experience of a young lady who memorized the 2000 most common Spanish words (without context) before she moved to Costa Rica. Did this memorization help her communication in Spanish with local Spanish speakers? Apparently not. Memorizing words is not a way to learn a language. It is way to increase one's vocabulary. Encountering words frequent times in real-world, meaningful contexts is the best way to go. Using an SRS software such as memrise can be a tool in that process. But so can regular handwritten flashcards, which also have the extreme advantage of having been written by one's own hand, thus incorporating tactile (manual) learning into the process.