I actually just happened to be hanging out downtown and found myself at a convention for people devoted to answering just that question!
DeltaFOSB is a gene transcription factor and is the precursor for the development of all forms of what is clinically called "addiction". However what the other commentators suggested [food is not addictive] is not actually true, it has a measurable affect here. In fact, Delta-FOSB can be and often is used as a biomarker of addiction in patients who are already deceased (and can have their brains broken open).
Now, as to the question of addictiveness, nobody mentioned anything about pornography there, but I did hear an interesting anecdote about the raw power that certain addictions have that clearly isn't present in pornography; Delta FOS-B accumulates in the ventral tagamental area and nucleus accumbens, two central regions of the brain, and many drugs have the action of creating floods of FOS-B precursors there, for example, this effect is so strong in heroin addicts, that they nearly always throw up upon recent re-exposure (sometimes newer Methamphetamine addicts as well) where the mechanism here is the flood of FOSB, which an outer region (she had mentioned but I have already forgotten which) interprets as "poisoning". So while it may be true that pornography is "addicting" - when is the last time you saw someone throw up when they looked at pornography (modulo ordinary "disgust").
What was doubly interesting was that she said the change in dopamine regulation (meaning the introduction of new dopamine-like substances + the change in the rate that your body "reuptakes" dopamine) in the most powerful addictions -- benzodiazapines and opiates, aren't actually that high, and compared with drugs like methamphetamine and cocaine aren't much different from marijuana.
It's this secondary affect that creates these changes in reasoning and reward structure with these drugs (she also mentioned that both have "withdrawal", which while technically unrelated to addiction is extremely painful and sometimes life-threatening).
However, to be fair, it was repeatedly stressed that the "secondary" affect was more than just Delta-FOSB. There were changes in signaling, new kinds of peptides that unaddicted people can't produce (???), something about phosphoration of camp ("new notes on camp") and bunch of change to "Dendritic Spiny Structures".
Among the living, questions like "drug-use reinstatement likelihood", "place conditioning preference" (the more addictive the drug, the more you want to use it in a safe, known place), increase in use over time, etc. are how this stuff is generally measured with reasonable economics.