This may sound silly to some, but you'll have to be a bit more detailed. I have never been much of a fan of Facebook thus I have no idea what such a feature actually does. Not knowing this I cannot analyze it to come up with any systematical way of recreating it in something such as PHP. There is quite a lot of ground to cover in an ambitious project such as creating a social site, but I suppose sticking to the topic, after a bit of elaboration I can probably help explain how something like a 'poke' feature would be coded for you.
"Given billions of tries, could a spilled bottle of ink ever fall into the words of Shakespeare?"