Why do I feel that my social network gets more and more narrow after working?

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I graduated this year.Now,I work in a private enterprise.But everything is not going as I imagined in the school.It's different.I have no opportunity to make new friend.All of my communication network expect colleague is old friends.Worse still,almost my colleague are married so there is no common topic between us.So I am confusing that how can I expand my social network?Can you give me an advice or you have the same feeling……