It is a lot easier to meet people in a big city. I live in New York City and am a member of a variety of groups where I could go out and hang with random people every night (and play trivia, board games, go drinking etc). Whether you'll actually like them and they'll turn into real friends is a separate issue.
Being able to walk everywhere, or most places, is a huge plus.
Not having as much of my own space (small apartment, no yard) is a downside for sure. The park is just not the same.
In the end, socialization requires effort and commitment either way.