Even though that subject is in the title, it isn't referenced directly in the opening post.
A lot of times, people don't stay in the same city/metro area as where they attended college. They take a job in a new city where they didn't spend a part of their childhood and didn't go to college. In those cases, you're a 22-24 year old in a city where you lack any social ties to that region. In those cases, you're going to have to make a new set of friends in this city where you lack social ties. You're going to be making friends with other transplanted adults with generally weaker social circles. I covered this topic in the "Observations on Dallas' Scene" thread. Excerpt from that below.....
"Getting into one of the social circles with Dallas natives with strong roots isn't that easy to do. It's a lot like getting into a top fraternity at a university. Most transplanted adults to Dallas won't get into those or they will be on the fringes of them. Additionally, a lot of the Dallas natives who go to local high schools, then possibly go to some unversity within Texas (maybe even Southern Methodist or Texan Christian locally) and then settle in Dallas as adults tend to get married earlier in life, settle into the suburbs, and have children. Some of those people might be your co-workers but you won't be spending your leisure time with them."
In the parts where I used geographically specific references to Dallas and/or Texas, substitute your own locality and the point still makes sense.
If you arrive in a new city post college and are still close to the age of recent graduates (up until your 30th birthday), you can still find people in your life situation pretty easily. Let's say you're a 32-36 year old unattached man moving to a new city where you lack social ties to the region. Do you think you're finding friends/a social circle easily? No. That guy is going to have to rely exclusively on some combination of swipe apps, social media DMs, or approaching strangers. The under-30 unattached guy arriving in a new city where he lacks social ties has a fighting chance of creating a post-college social circle, but it will be weaker than if he had pre-college graduation ties to that city/region. Many of the guys referenced in the previous sentence will become reliant upon swipe apps, social media DMs, and stranger approaches as well, even with that fighting chance of social circle creation.
Also, it's important to be aware of the medium to longer term (3+ years in a city). The problem with social circles becomes sustainability. Social circles have a way of getting pissed off at men who continually exchange girlfriends without marriage or babies, even if the relationships are semi-long (1-4 years). After 2 or so instances of medium term relationships, the social circle will run dry. Social circle is not likely ideal for serial monogamist who does have extended relationships but doesn't commit in a legal sense or the player type who tends to have relationships of less than 1 year.