I do not like charts like this because their existence implies that cost per user somehow matters, or is a deciding factor even, in the price of an acquisition.

Even if that was the major metric in determining a price, there's often a very large difference between the number of users currently and the expected number 1 year or 5 years out.

You don't buy a company because of the number of users it has right now. You buy it because you think you can do something better with it, or to be defensive in a field, or to expand your own customer lists.

What's more, not all users are created equal! When Flickr for instance was aquired, what percentage of users were paying users?

I think its hard to make any real generalizations here, but better than a chart of cost per user, I think this is probably closer to a chart of how badly Company X feels it needs some audience. It's some formulation of growth, panic, defensiveness, etc.