If the dead remain on social networks in perpetuity they'll make up a larger and larger % of the user base and actually have a significant statistical impact on the ratio of accounts vs. active users. It would take some sustained high birth rates or a dramatic decrease in death rates to reverse this.
In an extreme example, assuming every human ever born had a Facebook account there would be ~105B "memorial profiles" compared to ~7.5B "active users". Or, about 93% of accounts would belong to the dead.