Though reinforcement/mobilisation theories regarding the impact of the Internet on citizens’ political engagement are predictive, there are few longitudinal studies on how the profile of the citizens using the Internet for political purposes has changed and how this relates to such theoretical perspectives. Using survey data from four Finnish parliamentary elections, 2003–15, this longitudinal study examines the evolution of the predictors of belonging to the segment of citizens who extensively engage in searching for political information online during the elections. Additionally, the research longitudinally studies the evolution of the drivers of citizens deeming online sources as important for informing their voting decisions. In light of demographic and resource-based traits, a mobilisation trend is detected across time in the analyses. As to factors concerning attitudes and orientation to politics, however, a more evident reinforcement trend has emerged. These patterns are also evident when examining social media engagement through searching for political information during campaigns. Although the Internet and social media are becoming important for a demographically increasingly diverse group of citizens, especially the young, it is those already predisposed for doing so who have, over time, engaged politically to an increasing degree through these channels.