thought experiment:
Let's say Danny and I (who are in the same state, though separated by a zillion miles) decide to film a Star Trek series of YouTube videos of a pair of low-rank shuttle jockeys, making runs here and there. A star trek buddy sitcom.
We do it using a third TX ENworld friend's iPhone camera and the iMovie app.
That's presumably a hobby fan film project. Any money we spend on the set (spare bedroom somewhere and trips to the park for on-planet scenes) comes from us and we're not making any money. Most likely we are clear of the wrathful eye of Paramount's lawyers with our one subscriber (Morrus, who will watch anything Star Trek related).
Now consider evolution off of that.
Morrus likes it, tells his friends, and now we have lots of views. YouTube starts to send us a monthly check for our "revenue share". Let's assume it's modest and Danny blows it on ingredients for his next fancy dinner that I wasn't invited to.
Now, are we in trouble from Paramount? We just made money off our "fan film"
Now consider that we've been using an iPhone and the shakey cam effect has got to go. We hire a cut-rate pro camera man who happens to own a 3-gen old pro video camera to do the shoot.
Are we in trouble because we employed a "professional" to help with the creation of our Fan Film?
Let's say fan response to the improved video quality was high, but we could only afford that one episode paying for it ourselves. So we start a KickStarter to raise funds for 10 episodes, shot by a professional.
Now depending on how much we raise, we consider that we can also:
rebuild the sets/green screen to be better
hire some actual actors to play other roles
hire an FX company to do a better job on the intro/outro and CGI
hire a composer to score and record an original score for our episodes
replace Danny and myself with real actors who don't stare at the camera
At some point, this obviously smells less like Danny and I doing a hobby thing, and more like Danny and I arranging the production of a better version of our fan film.
Additionally, where initially we were the fans, funding our hobby, it could be said that now the KickStarter people are fans, funding their hobby of getting a star trek fan film made.
The obvious gray area is where us hobbyists doing the work and paying for stuff shifts to other people doing the work (and getting paid), and other people (kickstarter) funding the activity.
One thing to keep in mind, is that there is likely always a grey area. In just about every hobby, you are still paying somebody for supplies, equipment, services that you yourself can't do. That doesn't detract from it still being a hobby, even though you technically didn't personally do 100% of all the activities to get to the end goal.