Erik Duval asks for help. He points out that it is extremely difficult to figure out who cites him, how often, and so on. Using a tool offered by librarians (Web of Science) gives highly accurate, but also highly incomplete results. Meanwhile, Google Scholar fares better, but gives noisy data which overestimate how many people cite your papers. An astute reader comments on his article saying that you still have to take into account the blogosphere and other media.
My take on this? What tool actually matters is the tool other researchers use. If everyone uses Web of Science daily, and you are not there, then too bad.
Myself? I use Google Scholar. If you are not on Google Scholar, you do not exist for me.
How much impact you are having as a researcher is a fundamentally multidimensional problem. No researcher dominates any other researchers in every way. Trying to find one scalar measure that sums it up is futile, though it can be fun.