Being in a relationship definitely changes people's social lives, I'd like to think I still make an effort to keep in touch with my friends, but I have less time available now I'm with someone compared to when I was single. I work shifts so spend some evenings at work, and am an introvert so need at least one evening alone per week.
When I got into my late 20s my social life changed, I used to see some friends every week before then, and now it's more like every month, or sometimes longer. I have also drifted apart from some friends, some who I had been really close to but now are more acquitances rather than friends, no big fall out or anything, just drifted. One of them would always be sat scrolling on her phone like you describe in your OP, so I just stopped asking her to do anything, thought I'd see if she initiated anything and she didn't until nearly a year later when she messaged me saying she missed me, though she'd also just split with her boyfriend (suprise). Anyway, people also started "settling down" and moving to more affordable areas to buy a house. When we were all living at home with our parents, we lived within walking distance of each other, now it's a potential one hour round trip so meet ups need a bit more planning.
When I was single, I planned the week in advance, and would initiate more plans with friends than I do now. I was the only single one for a couple of years, and felt a bit lonely at times, but I kept myself busy with things I could do on my own. I career changed at 26 and then did a lot of overtime at work to save for a house deposit. The gym, running, yoga and swimming were very important to me. I signed up for a few charity events for running and swimming (can't believe I did them now!) which gave me something to focus on and work towards. I joined a walking group where I met a couple of new friends that I wouldn't have met otherwise. I also just started really enjoying my own company, if there was something I wanted to see at the cinema and no one was free then I would just go on my own.
Sorry for the ramble, your social life changing can be hard, I miss the times when I saw my friends more than I do now. It's a normal human thing to want to spend time with friends, we're pack animals at the end of the day (even though I'm also an introvert
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P.S. I'm jealous that you live near the beach!