粘 has two readings: nián ("sticky," the same character/concept as 黏) and zhān ("to stick to, paste"). For 粘性 the reading is nián. In Chinese tech/marketing, 用户粘性 is the standard term for "user stickiness" (engagement and retention).
mistakes
Some traditional-character texts and dictionaries write this as 黏性 (黏 is the more etymologically pure form). The simplified standard 粘性 is what you'll see in Mainland mainstream writing.
