Going off the comment hint on UnidentifiedX's great answer, you are a:
I look like a weapon if you know how to handle me (Edit)
You can use a story as a weapon against an argument and, even if you don't, your story can seem (look) like a weapon to others.
I am most popular than any other
Stories are popular ways to express an idea, transmit information, or amuse others. Technical manuals convey information and express ideas, but aren't popular unless they tell a story. Poems and songs that tell stories are often more popular than those that don't.
People use me often when they want to be informal
When people want to get to know other people, they tend to tell stories, which is much less formal than reciting a resume or CV. You might tell a story about a scar or a fun project you are working on, rather than recite strict facts about what happened.
Dirty minds see something lascivious in me
Stories can be told about a lot of things, and some people love to add double entendre's to make things more interesting. Some people hear them even when they aren't intended.
I am loved by road trippers
To pass the time during a road trip, people tell each other stories. Either it's in the car, bus, truck, or whatever vehicle, or it's around the table at a restaurant, around a bonfire, or just chilling before bed.
Some isolated people in the world have never seen me
Stories are everywhere, but are only seen when written down. There are still a few isolated indigenous people that have never seen a book.
I am man-made and when I am with my friends I can enlighten you
People make stories (ie. man-made), rather than animals, plants, or rocks having stories. People can tell stories about those things, but it's still the imagination of the human, not thing they are telling the story about. People often tell stories to give each other information or to help explain how a decision was or should be made.
I have trillions of twins, but they are mostly fraternal twins
There are more stories than can really be counted, and even with billions of people in the world, each one has their own large set of stories.
I am indivisible but I can become something else if you mutilate me (edit)
Most stories don't make sense if they are broken into parts. If you twist (mutilate) the meaning of a story, it can mean something completely different than the original intent.
I am made of love and I am made of war, guess what I am and became the next star
Stories are usually made by people who love to tell stories, and almost everyone loves to hear one. Many stories are about war, and war makes a lot of stories to be told as warnings to others. The right story can be made into a play, TV series, or even a movie, and the author can become a star if any of those options become popular.