Something similar happened to me - I was in a very tight-knit friendship group who all went to university, and I had nothing to fall back on. In my year, I haven't made friends with the people I work with (in that, I don't see or speak to them after work), although I am friendly enough with most of my colleagues so it gives me companionship when I'm there.
I didn't do extensive levels of travelling on my gap year, although I have gone on various trips throughout the year. Even 3-4 days in a hostel can get you meeting new people and a little bit of companionship for those days, even if you don't really connect with them once you go home.
It's worth saying though that even when my friends are home, I don't really go out and see them much - I'm happy to just speak to them digitally most of the time. Because of this, I haven't really minded the fact my friends are no longer in town - just having a bit of a laugh at work gets me through the day.