A costly signal, in evolutionary terms, is a hard-to-fake sign of expenditure of resources for the purpose of demonstrating mate value-- the classic example is a male peacock's tail. In terms of human behavior it could be any display of exertion or expenditure performed for a variety of reasons, but especially to vividly display

to a group or an ideal. A blog, by contrast, is practically the definition of

signaling: a place where ideas can be freely and rashly expressed without necessarily indicating

to anything in particular, but the messages are conveyed nonetheless.