...вас сработает:
if (100 > new Random().Next(0,100)) {
var elLike = instance.ActiveTab.FindElementByAttribute("button", "class", "like", "regexp", 0);
instance.ActiveTab.FullEmulationMouseMoveToHtmlElement(elLike);
instance.ActiveTab.FullEmulationMouseClick("left", "click");
}
else...