Yes, there are two ways to do this.
First you should know that Unity’s regular model pipeline is part of the editor, so it's not included in your runtime. You’re going to have to work around that fact.
The easiest way is to import the model into Unity through the normal pipeline and get it all set up, then build it into an asset bundle, then host that asset bundle on the web. Then your runtime can just pull down the asset bundle from the web and load the resources from it.
As I said, that's the easy way because you've got it all set up properly in Unity first and you can make sure the materials and textures and animations everything else get packed up with it in the asset bundle.
Here's the manual page for that.
The other option is to use a script to import an ASCII encoded FBX file you pull down from a WWW source as text. This is harder because you have to use someone else's script, and you also have to fetch textures and other things and get all the materials and animations set up.
I've never done it this way myself but there's plenty of discussion out there about it. FBX Mesh Importer At Runtime
If you have all of the models in advance then you're going to want to use the Asset Bundles route. If it's something where the user is supposed to supply the model then you're going to need to figure out the importer script.
I've got a fair bit of experience with Asset Bundles, and you can do some pretty tricky stuff with them but they can also be pretty fiddly.
One nice trick is that your Runtime doesn't need to know what Asset Bundles are available in advance, you can just set up an API call to pull down a JSON string with all the available bundles and their URL’s. This is great for something like auto populating a micro transaction based store where they can buy costume pieces for a character or something.