On a person's profile, it will state when they were last online. The max you ever see is "two weeks." Except it hasn't been two weeks. It could be months or even years. Worse yet, they could have cancelled their account!
They keep these accounts active WITHOUT YOUR PERMISSION to "up" their numbers so more people join. If you really knew how few people were on the site, you would never join. So they fake it.
Great way to start a relationship. Just sayin
Pros
Not a damn thing
Cons
Too many to list