Facebook uses 'suggested pokes' to try and get users to engage with users they might not have engaged with if they hadn't been prompted to.
Why do Facebook use suggested prompts?
More engagement means more pageviews which means more ad views which means more money. And Facebook have every intention of making money.
How do they know who to suggest?
Facebook doesn't disclose the algorithm it uses, but I wouldn't be surprised if it employs something similar to ad retargeting: Where Facebook monitors what you do organically on their site, and then they show you "calls to action" (click this, view this... Umm... Poke this) based on that activity.
It's worth noting though that, due to the general "perceived creepiness" of pokes in general, that feature of the site has become far less prominent and needs to be activated by the user in order to see it.
