My first response here was to explain in this circumstance a person with "strong social skills" would have a number large groups they interacted with. For example religious groups, exercise groups, charity groups, etc.. All of which could provide a handful of potential mates through casual encounters. So easily a few dozen opportunities given a reasonable sized city.
But... that really implies there is something wrong with people who don't have the time, or inclination to involve themselves in such activities, or simply aren't the kind of people who can successfully get a date from a casual encounter (heck I've known people who get get dates from 30 second interactions in the grocery store) like this.
So, while I'm far from an expert on this, I suspect this is just more of our modern sickness. The idea that not only does a person have to fit a long list of criteria to be eligible, but they also have to have honed their social skills to the point of being able to woo someone into a date with just a casual encounter. And frankly, the people I know who could accomplish this were also some of the "worst" people I knew (lets start with the idea that they were completely unreliable lairs).
So, I return to the idea that traditional human society weren't these huge cities, but rather smaller villages where it was possible to have such a limited pool as to generally grow up with your spouse, and be aware that a large part of bringing up children/etc was spending time on those lower rungs of the ladder working out issues. To keep going with the idea that someday in the future it would be possible to climb back up those stairs to happiness. But this also means that such modern conveniences as birth control are actually harmful to society because its possible for a couple to fall into/out of love without the traditional responsibility of raising the children that were the result of a few weeks of biological romance (and lets face it the lusty rutting of 20 somethings is little more than biology having its way).