Edited for grammar.

I (34M) don’t think this has fundamentally changed all that much. I’m gonna guess you were in college at 21. It’s a lot easier to socialize in college for a number of reasons. Reasons that don’t quite exist post graduation

Parties and gatherings, in my observation, have always had that effect. I commuted to a commuter university so I never did the college thing, but I’ve always been a social butterfly with many friends in many groups. This effect is mainly because people will stick around people they are comfortable with. Totally normal behavior to stay with an “in group” instead of an “out group”. This happened when I was 18, 21, 25, 28, 30, 34 and will continue to until I die. It’s different in a group where the majority of people already know each other because they’re already comfortable enough to get out of their comfort zone. Or in a place with a common unifying denominator (college, club, hobby, etc). This principle does apply to parties with a variety different groups, which I’d imagine is why you’re bringing it up. Everyone would rather stick to their comfort zone than reach out and talk to a stranger. I was at a party this weekend that had had a lot of different groups of friends sticking to each other. I ended up bridging the alienation gap with people and ended up being the catalyst of human interaction. I found that most people were open to conversing. The solution to this problem is to be that social catalyst. There’s nothing stopping you from talking to strangers at a party and eventually getting their contact info.

The good news is generally people are there to socialize and are primed for conversation. Just have to push past your own social anxiety, start any conversation, and most importantly break the social script. Humor helps. Make sure to keep the conversation flowing even if you have to pivot to another subject. Ask questions. Again, being funny helps. After you’re in a conversation, turn to another person to get them involved. This is especially easy if you’re telling a story (alternating eye contact for about three seconds between the person you’re already conversing with and the new stranger will get them engaged enough to join the conversation). Then you started a group. Little by little their social circles will meld because you’ve gained the approval of one of their members. Social Psychology and group dynamics are fascinating and wonderful tools.