I thought of announcing I was pregnant, it being SO absurd, since I'm alone in the woods with NO man, ha ha, but then I realized a couple of long distance friends would really be thrilled for me. I'd feel like a jerk saying "just kidding..." so I skipped the idea.
Back when I had a boyfriend, I announced we were engaged, and updated the relationship status to reflect it. Not surprisingly, everyone freaked, my cell was ringing off the hook, etc. Until I announced it was a joke a few hours later.
I posted a complaint about the weather report -- that we were going to get yet another round of late snow. This was a good prank, because it DID snow about a week ago AND we are getting a night in the mid 30s later this week. This is despite it being in the 70s now with predictions in the 80s for upcoming days...
And, given how much I've griped about snow all winter, it was believable...