I think that Ihtkwot has answered the first part of your question very well. I'd like to address some of the assumptions in:
If so, is this chiefly a reflection of a change in the technology and role of Naval warfare in the last 100 years, chiefly a lessening of the USA's ability to project military power in the world, or a combination?
There isn't a simple answer to this question, and if there were a simple answer it wouldn't be any of the assumptions within the question. Since WWI, technology has changed, strategy has changed, and US national and foreign policy objectives have changed. The changes haven't been continuous; they've been disruptive. The Fleet we needed for WWII is different from the fleet we needed for Korea, which is different than the fleet we need today. There isn't a smooth line on a graph which describes the change.
Fewer ships does not mean a reduced capacity to project power. If you glance at the tables that Ihtkwot provided, you'll note that in 1917 we had 0 carriers, but we had 66 destroyers and 37 battleships.
Between that time and now
- Naval Air Power changed the technology by which we project power. 1917 battleships were able to dominate the sea and a strip of land adjoining the sea. Modern Carrier Battle Groups can project power over essentially the whole globe. We've restructured the fleet into Carrier Battle Groups - that is a completely different set of requirements than the WWI Navy.
- Strategic Objectives - Again, as ihtkwot mentions, the US Navy is the only blue water navy today. Prior to WWI, there were a few nations that were competing for naval dominance. There is a vast difference between sea control, sea power, and force projection and the forces that need to be constructed to address each of those.
In 1917, the United States had far more farms than we do today, but today we eat far better than we did then (both in terms of the number of calories consumed and the diversity of foods, and probably in terms of nutritional content). Simple comparisons are precisely that - simple.
What size fleet is sufficient? What do you want to do with that fleet? Is the objective to preserve our freedom to navigate the sea against the dominance of other Naval powers? Is your objective to fight pirates? Is your objective to defend western democracy against Communism? What is the risk that the US will be pulled into a Naval conflict off the coast of China (Daioyu/suyan/etc.)? Do we need to prepare a Navy to support our allies in that region? What is the cost to the country if we fail at any of these missions?
Right now Naval Staff is actively and vigorously arguing what the best force structure is to fight asymmetrical warfare/terrorism. Should we build more Hospital ships? More Littoral Combat Ships? More destroyers loaded with cruise missiles? More Landing Ships to move Marines in and out of regions? Or do we revitalize the doctrine by which which deploy carriers?
These aren't simple questions and it annoys me (I suspect I'm not hiding my annoyance well) when political debates pretend that you can evaluate the Navy by stacking up ships like lego blocks; my stack is bigger, I win!!.
Please note that my annoyance is directed at the political debate, not at the original poster, or anyone else on this board - I'm trying to respond to the second half of his question.