circulating microRNAs are functional?
See papers by D Bianchi for reports of miR levels in blood of pregnant humans, both of maternal and fetal origin.

A review by Steer and Subramanian offers a number of examples pertaining to your question.

Lastly, Vickers, Palmisano, et al. (2011) show that some miRs are present in the HDL-cholesterol particle and that those levels differ between normal and familial hypercholesterolemia human subjects.