Passive data from a tracking application (or "meter") installed on participants' devices to register the URLs visited have great potential for studying people's online activities. However, given privacy concerns, obtaining cooperation installing a meter can be difficult and lead to selection bias. Therefore, in this article, we address three research questions: (1) To what extent are panelists willing to install a meter? (2) On which devices do they install the meter? (3) How do panelists who installed the meter differ from those who were invited but did not install it? Using data from online non-probability opt-in panels in nine countries, we found that the proportions of panelists installing the meter usually vary from 20% to 42%. Moreover, 20-25% of participants installed the meter on three or more devices. Finally, those who were invited but did not install the meter differ from those who did.