I can tell you that from an Austro-Hungarian view, military service was not really the popular route for the nobility by the time of the first war. I'll have to specify "real" nobility, as there was a practice of "ennobling" long serving high ranking officers as a reward for their time in uniform, and the title would be hereditary. So, Franz Conrad "von Hötzendorf" had a noble title but, really, he wasn't noble. He was great-grandson of Franz Conrad who earned the "von Hötzendorf" appellation via this military service route. This type of nobility is called the Deinstadel ("service nobility"). They are not who you are interested in.

Of course, the Austrians had Archduke Joseph Ferdinand, a poor-to-mediocre general who initially commanded the 14th Corps against the Russians, then commanded the 4th Army. Archduke Joseph, an apparently competent general who initially commanded the 31st Infantry Division and then the 7th Corps (mostly against Italy along the Osonzo. Archduke Friedrich, who "commanded" the army but really was expected to stay out of the way of Conrad. Archduke Eugen started the war with no active command but was given the broken 5th Army after the opening Serbian debacle and ended up doing a reasonable job commanding much of the Italian front.

However... as of 1896, less than 14% of regular army officers were of the real aristocracy (excluding Deinstadel). This percentage continued to decline as the war approached. Further, 89% of general staff officers were of middle class origins and 75% of those of General rank. Even among the most "aristocratic" force in the army, the cavalry, only 9 of 42 regiments were commanded by actual "nobility" by 1914. So the overall numbers of nobility in the military were dwindling. The Austro-Hungarian officer corp had a significant element of "caste" to it, where many of the new officer candidates were sons of older officers, and most of those who were not of military families came from the middle classes not nobility. The noble sons of Austria had been finding more interest in industry, finance, and arts than military since at least 1867.

There were examples of nobles, even high ranking nobles, fighting at the front though and not just commanding from some theater HQ.

Archduke Karl Albrecht began the war as Oberleutnant in an artillery regiment. Although he was pulled from the combat area for general staff work by the end of 1914, he did return to command an artillery regiment for 1916-1917 which is still what I would consider a combat area command. Then he commanded the 23rd Infantry Brigade until the end of the war, which I would also consider a combat area command as regimental commands could easily find themselves in the action.

Archduke Joseph Franz entered the war in 1915 as an ensign in the Honved 7th Hussar regiment. He remained with that regiment for the course of the war, being promoted in early 1918 to Rittmeister (equivalent to Captain for a US/UK analog).

It's certainly possible politics played a role in the various medals and promotions these two had, but... they were still there with the unit!

So the answer is, yes, nobles did fight at the front... just, not that many of them, and certainly not as many as you might have seen 100 years before.

Sources of interest

Fall of the Double Eagle by John Schindler

The First World War: Germany and Austria-Hungary by Holger Herwig

There's also a couple web sites that... well, I wouldn't cite them blindly in a paper but they are really good for leads or tid bits. I would be very sad if either site would disappear.

www.austrianphilately.com has some incredibly niche detail that has been hugely useful to me in a project I've been working on. I'm already talking about Austro-Hungarian history of WW1, so this is niche information for a niche topic. It's a hairline fracture in history.

www.austro-hungarian-army.co.uk has much info as well. They list a large bibliography, but often don't specify what info comes from what source.