On social network sites (SNS), people are increasingly confronted with news content—even if they have not actively been looking for it. Although it is widely recognized that SNS have become a main driver for such incidental news exposure, we know little about the factors that influence whether users engage with news encountered on SNS. Thus, this study investigates under which conditions incidental news exposure becomes actual engagement with news by asking how both the perception of the news post and general news usage patterns influence the intention to read news articles encountered on SNS as well as the intention to look for further information about the covered issues. Building on a mobile forced experience sampling study consisting of 840 Facebook news encounters reported from 124 participants, we find that news engagement is mostly determined by participants’ perceived interestingness of and prior knowledge about the issue of the news post and to a much lesser degree by social factors unique to SNS (i.e., feelings towards the spreader of the news). In contrast, no influence of content-independent news usage patterns on news engagement could be observed.