I'm just wondering if they are worth it? How are developers monetizing these apps?
I think at the very least I'll buy a couple of domains, throw up a landing page and collect interest, but would enjoy hearing your feedback.
Monetizing becomes - ad network (rockyou, adsense) or selling virtual currency through offerpal/superrewards, etc. There is a section in the developers forums that talks about monetization (http://forum.developers.facebook.net/viewforum.php?id=8) and mentions all of the latest players. Offerpal was super on another app I did - it just didn't get the traction it needed to make more than a handful of bucks.
If you go with a completely viral app that isn't a game, you don't get the return visitors but you might get the mass numbers. If you can get into a vertical niche, you can go to connectcommerce/cj.com or something similar and run targeted ads for affiliates.
I did a quick two page survey app that hit 40k DAU at it's peak and has about 400k total installs that made roughly $800. Not a bad return for 40 minutes of work, pylons/nginx/uwsgi and a small database. Replicating that with different angles never met the same success. Timing probably was just lucky on that app. Once in a while a small uptick on adsense comes in - and someone has stumbled across it and their friends all start taking it again, but, never with the original zeal.
I have an idea for a social game that has a different twist. While I found I could write software, knew the psychology behind it, I found out I'm not a game designer. I wrote all of the code but lost the motivation to finish it.
It works, you can make money at it. Writing a Mobwars/Mafia Wars clone is the quickest way to money. However, designing something viral just requires a lot more traffic to make the same cash. Neither is incredibly difficult.