Like friend requests, you're allowed to "poke" anyone (it shows up privately on that person's home page as a visual icon). But just because something is allowed doesn't mean it should be encouraged. With strangers, poking is the digital equivalent of eyeing someone across the room without working up the courage to say "hello." With friends, it's a quick way to remind them you exist.
Thoughts on Poking
Never Poke
Please don't. Let's be real, poking in the flesh is lame, so how can virtual poking possibly be cool? If you must poke someone, please God don't think it is an acceptable form of flirtation. It isn't. Instead of endlessly Facebook-poking that super cute boy in your biology class, or your anthropology class, or whatever, how about you actually DO something with him. Coffee? Tea? "Study" party? 'Nuff said.