Every little thing we do has some effect, from smiling when you say hello, to holding the door for someone. The effects may not be long lasting-but you affected *that* moment, a moment that will be gone forever. So, even if you believe your own life is meaningless, you're either contributing to that moment in time or not.
To me, the fact that we're all unique means our lives are worth something. No one else thinks exactly like you, reacts the same, or has the same knowledge base and experiences as you. So, in that capacity, no one else can provide to the world what you can... thats your "meaning" to the world.