[disordinary]

We're already seeing governments like Russia imploring people to reproduce and giving incentives to do so. Also low birth rates in developed countries are in-part affected by the need for both parents to build careers, if the world economy collapses because of declining population (which results in a lack of growth and also soaring healthcare expenses) then we'll probably see reproduction increase again - especially if coupled with the above mentioned incentives.

[ralfd]

Birth rate will never decline to zero, as the next generation will be biased more to whatever it was what made their parents to get kids, and less to what made people childless. For example in the extreme case that Facebook would make people sterile, than some day simply the Amish will rule the US.