In the VFX environments I've seen (100+ artists), most artist workstations and servers are Linux (usually RHEL or CentOS due to much higher rpm than deb support in the industry). Macs are common for editors (who use the Adobe suite to make timelines for dailies) and producers (because shiny), while Windows is mainly used by admin staff (accounting, hr, etc) because they use the same terrible accounting software as every other company out there (note: this may be less so now with the move to web based solutions).
While most hobby level video software is targeted at windows or Mac, the truly powerful stuff (houdini, maya, nuke, blender) are first class citizens on Linux. Currently one of the few artist programs that aren't available for Linux is the adobe suite which is mostly used by editors (for timelines) and photo editing (for promotional material) and most of that happens on Macs, usually in the editing department.
Something to keep in mind is that many VFX shops are kind of specialized and will work on very specific portions of a movie or TV show. For example, the company that owns the final product (Disney, Fox, MGM, etc) will pay company A to shoot the movie, then company A will send the footage for a 30 second scene to company B to rotoscope everything, then it gets sent to company C for VFX elements, then to company D for 3d conversion, then to company E for lighting and finally to company F for audio and final stitching. This is repeated (often with different companies A-F) for each scene since different scenes required different levels of expertise in different areas such as crowd generation, fire simulation, raindrop removal (yes, it's as painful as it sounds), background replacement, CG helicopters, etc. Because of this, the few steps that require windows are typically handled by 1 or 2 companies so the rest only need enough Mac/Windows machines for their local editors to stick timelines together for daily reviews.
An interesting side effect of the above is that almost every company you see in the credits of a movie had no idea what the dialog was because audio is one of the last things that gets added and completely bypasses 90% of the VFX houses involved.