Katja Grace has a great post at Meteuphoric called "How Does Information Affect Hook-ups", where she looks at the impact of online dating and the free up-front exchange of information about partners that it engenders, as well as the impacts thereof.
In a comment I argued that this was one example of how increased market efficiency has changed the romantic behavior of humans. Sometimes increased market efficiency is the result of information technology as in Katja's example, but it can also be the result of urbanization, as I argued previously.
In response to Katja's post, I argued that if we want to see the effect of efficient markets on dating, we didn't need to wait for online dating - single people in dense urban centers were and are exposed to many, many other potential partners over the course of a day, and even though they don't get the same up-front information that online dating allows, the effect is that people have a much better idea of the overall market (even if not necessarily an immediate read on the quality of the individual across the dinner table from them). In a little town in a rural area, you can't take individual dates too lightly, because how many single people in your league in your age group are there going to be out there? Contrast with Manhattan or Tokyo, where on every date you can tell yourself well yes, s/he is attractive and pleasant and interesting, but that guy/gal you just met at a professional mixer last night seems even better, and if neither of them work out there are still (literally!) a million other potentials out there. Given that dating is a usually required step before marriage, one would expect this is contributing to later marriage ages in dense urban centers, although there are confounding factors: a) urban centers in the developed world have large numbers of highly educated high-earning women; women often seek men with greater education or earning than themselves; therefore, they may hold out longer for this reason, and b) that education takes time, and people haven't even started the earning phase of their careers when they're 24, much less are they ready to get married. Comparing urban to rural marriage age while controlling for education and income would tease this out.
A further confounding factor is the partner-finding-process itself, which varies considerably across countries - hence the mention of Tokyo (arranged marriage is still more common in Japan than Westerners realize). Consequently it would also be interesting to compare arranged marriage rates in Tokyo to outlying areas of the country. A personal observation of Asian friends who've accepted an arranged partner is that they seem to view one of the biggest benefits of accepting an arranged partner as having been relieved of the task of the search. This theory's prediction is that in a large and efficient dating market, rationally optimizing singles should be less inclined to listen to bothersome parents who want to arrange a marriage with the down-the-street neighbors' nice but homely son or daughter from back home. (Also known as "Mom, are you actually serious?")
Notably, Katja's post is about hooking up, not dating/marriage. She states an apparent conundrum, that people want partners equal to or better than themselves, which leads us to wonder why people hook up at all. One answer is that people have different valuation criteria and they look for different qualities (otherwise very little exchange of goods and services in general would occur!) Hence, romantic partner's frequent "joking" that s/he got the better end of the deal in the relationship (one hopes the other partner is not so self-deprecating that they agree!) This argument extends to whether people are looking to hook up or to date seriously. In particular, people looking only to hook up might not be so concerned with having a higher quality partner. This is where your own anecdotal experience of dating in a big city may be helpful to understand the phenomenon: you might have had a relationship with a good-looking successful guy that you thought was going somewhere and was quickly ended by him, for no reason that you could figure out (reason: he was never serious and it was doomed from the start). Or, you might be a young guy pleased with yourself that you were able to get a really hot, smart professional woman (often a few years older) only to realize after a few nights together (and a few oblique comments about a recently departed ex) that she's not returning your calls anymore (I dub thee "transition man", and I share your pain or at least your ambivalent feelings about the situation). It is likely that these encounters would not occur if both partners were looking for the same level of commitment. Traditionally women are more likely than men to express interest in romantic relationships that go somewhere, rather than in flings, although the latter situation has become more common.
It's worth mentioning that in tolerant large cities there are also questions about same-gender hook-ups and relationships, but my comments are restricted to heterosexual pairings since as a straight male my experience and reading has been about that realm. The underlying assumptions of the dynamic (male vs. female reproductive strategies, dad vs. cad, short- versus long-term mating females, seeking genetic quality for future reproduction) may not apply or have very different effects in same-sex relationships, though these relationships may also provide a unique venue for testing these theories.
If we expect that access to information and market size affects dating and marriage, it should affect reproduction and gene frequency and distribution, which is what I was posting about before. I think that exactly this effect can already be observed to have worked more in some areas of the world than in others.
As a final speculation - because our romantic lives are far different than those of our paleolithic ancestors, and there are probably paleolithic-diet/mismatch hypothesis type arguments to be made about widespread relationship and family problems that we see in modernity; i.e. is divorce rate in industrialized countries the obesity and diabetes of modern relationships?