I'm looking around for any halfway decent research on loyalty. Google gets me more "brand loyalty" than anything else, which doesn't really count as loyalty to me.
Loyalty to causes are a bit shady too.
Loyalty to other people within a relationship means something, though. The entire fuedal system was built on formal relationships between lords and vassals; to the extent that one was loyal, one actually stuck with that relationship. The vassals obeyed, but the lord was also obligated to provide.
So being loyal means self-governance based on the promises you've made in your relationships. You can do something similar for a cause or a brand, but there won't be the feedback- a cause can't be loyal to you.
This is one part of building strong communities that I see as unmistakeable:
The more relationships within the community, the stronger the community. In fact, I think the relationships are the community, with the governance and the form of the community coming out of these personal decisions.
All of which makes understanding loyalty important, but I'm not even sure if anyone has it properly defined yet.