Are these plugins sending every visited web page to their servers, or is this done when the user clicks/hovers on a name/profile?

I can't talk about others. I'm the founder of Vibe. We take privacy very seriously. We don't read anything more than we should. All we do is 'mark' the emails right in your inbox/page. When you hover/click an email, we send just that email to the server and returns the data,

Vibe just uses FullContact (see their FAQ) who's data is pretty incomplete.

Connect6 uses LeadFerret to power their plugin, which is even worse.

Hey Priley, Vibe uses FullContact. However, it is not just FC. We have other sources, our own crawlers and APIs as well. This is why we are steadily getting improvement in our data. PS: I'm the founder of Vibe.