I dont know if the ingame add screen has the same value as other RL adds. I often use adds just for fun. It one of the reasons they took out the add terminal from PA. They dont want noobs posting about pet societys and such, so im am guessing it is worth it. I often see players looking up at the screen. There are many ways to advertise in game. I think the add screen is the best. Unless you place an add in the log in section. You have to look at it there. I would place an add in a country wide venue. Any add you place now will just be a seed. If its on the ingame screen its still plants seeds. It takes only one person to see it and join. That one person could have value in the next 5 years. So even if EntropiaPlanets has two viewers I would pay to advertise here. Its fun and yes boath ingame an inforum are good places to work from in this goal.
in short, NO. the current ad system is not worth spending money on. SFE have not been heavily advertising for that very reason, also that PC stuff can't be used. The whole global advert system needs an extensive overhaul (i've still got an open support case just on PC uploading which is the reason why there are several blank signs in each of our shops).