Progress over time: The law of diminishing returns?

I haven't posted for a little while. Partly this is a result of other activities keeping me busy partly reflecting about the graphs below.

Essentially, I think all our optimisation efforts are hitting a point of diminishing returns.

Graph 1 below shows the percentage of traffic generated from a range of sources over the last 12 months. Amongst other things it shows a useful reduction in reliance on pay per click advertising and people knowing about the site and entering the name directly (probably mostly staff). It also shows that over time search engines are now bringing in 60% of all traffic while directories and other related sites are bringing in a lesser proportion of visitors (although in absolute terms the numbers are up).

Graph 2 shows relative performance against a baseline of 12 months ago ie today our traffic numbers and total pages visited is nearly 60% more than a year ago. This is quite an improvement. Total visitors and pages viewed track closely as over time the number of pages viewed per unique visitor has not changed significantly. Unfortunately the other trend that can be observed from the graph is that the rate of improvement is slowing down - we are getting less improvements from our incremental effort.

Perhaps we are reaching the point where incremental effort is not justified. Overall ranking on search engines etc is improving as is shown by the relative improvement in traffic generated. There is still more work that can be done there but perhaps a fundamental shift to our approach is required to lift performance to another level. Have you experienced this phenomenon with your web optimisation effort? What did you do to break out of these limits?