I can say that I myself have lost interest in Minecraft multiple times, but that has been merely upon the case of having played on it alot and actually feeling a need for a break from it. Turns out after having had a couple weeks break i begun to come up with new ideas and things to do, and then began to play on it again. I think it is wise for everyone to take a break every now and then, do something different, then after a while come back and have a new experience. Some times trying out different servers, adventures, etc can keep you entertained but alot of the ones i have been associated with you can only go so far before you begin to get bored with it.
Possibly a good bit of advice for people who own servers is to actually capture peoples attention in a good way and build their server in a way to which people have alot of options of things to do and people to play with.
From personal experience single player is pretty dull most of the time, but when you go on a server it is usually a more active and social experience, but even that is usually limited to the server creators experience and creativity.
I have ran a server with my best friend for a couple years, and we ended up with some faithful regulars and then the odd groups etc, and alot of what we did was all learning, with learning how players react and interact in different circumstances. At the moment our server is down due to us not having the funds to pay for the hosting, but we are currently using this down time to upgrade our server experience so that players can truely enjoy as much as possible and be able to have a really good time.

All in all, everyone's minecraft experience online and offline have their similarities and differences, but everyone has to take a break at some point and just refresh themselves, then when they come back they will be full of new and great ideas for a new experience so to say.