Facebook joined the Linaro Enterprise Group (watched the announcement at ARM TechCon in Santa Clara last year), indicating that they are at least interested in developing that necessary server environment.

You're definitely right there is a large software maintenance cost, but that has to be weighed against any perf/watt savings (Anandtech's Calxeda review shows even early systems have potential http://www.anandtech.com/show/6757/calxedas-arm-server-teste...), being able to put pricing pressure on Intel, and not being tied to Intel as a single supply source.