Are questions about online 3-d printing services allowed? For example, if someone needs a prototype and is not so concerned with the actual process of 3-D printing, and the question is around the 'best' online service or recommendations of which company could create the part (based on size, type, or financial considerations) considered on topic?
As with your other question, I think the same rules apply. Typically you want to stay away from asking questions about "What is the best...". Instead, try to present your question as more of a "What is the best... for me?" type question. As I mentioned in your other question, try to provide more details of what you might need. In the case of 3D printing services, I'll use the example of 3D Hubs and MakeXYZ (I'm not affiliated at all, just an example).
A "What is best..." question may be appropriate if the contents of your question contains key characteristics of what you, as an individual, needs out of the service. For example:
I need a part printed, but I also need this key feature machined and coated. I've looked at 3D Hubs and MakeXYZ, which (at this point) would be able to perform all of these operations for my part?
On the opposite perspective:
I want to start providing 3D printing services, but I have a CNC router and some other woodworking machines that I'd like to provide services for as well. Which might be better for me, 3D Hubs or MakeXYZ?
In each example, there is more of an emphasis on what you need as an individual. Another user can take the answers with a grain of salt and best apply the answers to their own situation in the end.