FBinNY 

I've always felt that infrastructure is built to accommodate existing travel patterns, rather than to drive them. There are many factors that determine whether people will take up bicycle commuting, including cost, convenience, distance, age, and life situation.

With exceptions, cycle commuting is the province of young urban singles. When folks marry and have children, family needs often make the bicycle lifestyle impractical, or at least more inconvenient, so people adjust. They may also move farther from work and then the bike gives way to alternatives.

So it shouldn't surprise anybody that the number of cycle commuters plateaus, as most of all likely to do so are, and others aren't interested no matter what the infrastructure is.

Also, consider that there's an attention span (for lack of a better word) factor wherein people take up cycling, do it for a few years than move on to something new. This has always been a few years for cycling, and it makes sense that it's similar for cycle commuting. So for a few years the idea is new, and trendy and lots of people take it up. After a while the novelty is gone, fewer people are taking it up, and more people are dropping out through normal attrition.

However, the plateau or slight reduction isn't necessarily a reason to abandon infrastructure. It's more a matter of setting reasonable expectations and goals, and basing decisions on that.