Again mostly personal experience and direct feedback from people saying that keeping track of vocabulary is hard. Lingq was an early inspiration, but it has ma

Again mostly personal experience and direct feedback from people saying that keeping track of vocabulary is hard. Lingq was an early inspiration, but it has many limitations (for example, it has trouble tracking word conjugations, let alone kanji in languages like Japanese) and relies almost exclusively on manual volunteer work. It provides very little leverage to users. For a purely leverage-based answer, Parsnip will save hundreds of hours of manual work per user purely by automating manual things like creating fill-in-the-blank exercises, vocabulary tracking and dictionary lookups (a conservative estimate is 70+ hours for this alone).