I don't much like the acronym 'KPIs'. It's because of how the term is misused, and also because of how the idea of KPIs so often leads to a misuse of web analytics.
KPI (usually) stands for "Key Performance Indicator", and for me it means "a number that indicates success". It's really rare to come by a true KPI, because it's really rare that a number should be so unambiguous that its going up or down, by itself, means either success or failure. Ten-year net profit (£) is a reasonable KPI, in my book; but then, should it be change, month-on-month... or deviation from target...?
The difficulty here is that metrics must always have context; and without some context, of other metrics, and of human knowledge and intuition, a metric can be ambiguous, uninformative at best, and easily misleading.
And yet still we must boil down web analytics to a spreadsheet. We must have reports! We must have 'KPIs'! Why is that?
Actually this dependency inherent in virtually every marketer is not such an unreasonable demand. I provide excel dashboards for my clients, highlighting a few key - metrics. It's because, with all the detail available, web analytics can feel a bit like being faced with an aircraft cockpit.
In order to know that we must act quickly - that something's going wrong - it's important to have reporting that is effective, but also simple. As Mike Teasdale will frequently remind you, when a pilot needs to know in a hurry whether he's going to suffer a mid-air collision, he ignores all the dials and judges relative trajectories by a speck of dirt on the windscreen...
For as long as I've worked in digital marketing (nearly four years), reporting has always been focused on conversions, CPA and RoI; and that feels simple, because we all understand the notion of revenue driven as an obvious measure of success. The problem with revenue and digital marketing though is that for as long as we make the most of our ability to track behaviours online with cookie-based data, rating all our marketing efforts solely with attributed revenue is neither totally effective, nor at all simple.
Not everything drives revenue directly! Stop trying to report as though it does!
This is something that needs discussing separately, in its own right, but the important point is that by studying measures of reach and engagement, as well as revenue, you can allow each of your different marketing efforts to be credited for the jobs that it does.
Banners get credited for driving awareness and engagement; social media engages your audience, and gets credited with the value it deserves; different parts of your website's content get credited with the different types of value that they drive; paid search and the web checkout process still get credited with sales.
And if you want, you can put pound signs next to your engagement metrics - seriously! Plus, there aren't too many measures of reach and engagement - you can put them all on the spreadsheet :)