I think the idea of a soulmate is an outdated, spiritual concept. Besides, it's depressing if true, and it's depressing if it's not true. Allow me to elaborate!
If the concept is true, well that means that everyone in the world is destined to be with one other person. Now, let's say that someone starts dating someone that isn't their soulmate. It's not a perfect couple, but they've given up trying to find the "one", so they get married. BOOM, now they'll never be with their soulmate, and they've both left their soulmates stranded as well. Of course, that's a silly theory, because I think that soulmates are silly.
Now, if the concept isn't true, it's a little depressing, because it means that we actually have to do some work instead of the universe. I'm not saying that there isn't someone out there who's "perfect" for you, but I'm just saying that it isn't predestined. I like the idea of soulmates not being real, because it gives life meaning beyond relationships.