DILEMMA!! *insert quandry type da-da-duuuuhhh music here*
Solved... you type a cheery "LOL" underneath their post and carry on with your social networking and the rest of your life. Now you're safe! You've acknowledged their post, you've made them believe that you share a sense of humour and you're not about to be dumped from their friends list for not "speaking" to them for a while. Everyone's a winner.
*disclaimer* The above post does NOT count when I type "LOL" on someone's post. I only post if I think it's funny. Or something like that... *whistles nonchalantly*