I applied too but finding out that it is also based on Forum activity is new to me. Surely the more you are on Forums the less you are actually playing the game?
Yeah, the whole point of this beta is so that people can test it, then report the bugs and glitches they find here on these forums. I highly doubt forum activity will play a part in the selection process. That just seems like something Bethesda brown nosers invented because that's what they want to believe.
The DLC releases in a little more than 2 weeks. So they are going to want this beta tested thoroughly in a short amount of time. Which means they will be selecting people who spend a lot of hours gaming per week. Also, they will be selecting people who have a lot of hours of game played in conjunction with only a few character builds. As this shows not only that are willing to put time and effort in to the game, but also that you finish what you start. They need this beta tested start to finish. Someone who flops around and has 30 character builds but never finishes anything isn't what they need in a tester.
An other thing they don't need in a tester, is someone who just goes around posting about how Bethesda is the second coming of Jesus and you pray to them every night. And how your son's first name is Todd, middle name Howard. That does nothing to help anyone's case. An other likely factor is, they are more likely to choose people who had an existing account at the time the beta was announced. This shows they are most likely to be on the forums reporting glitches, rather than just trying to get free DLC early.
Keep in mind, these are just theories based on logic and common sense. I don't know anything more concrete than anyone else. Though based on past experiences, I'm usually right about these things. But hey, I have my fingers crossed for a beta code too.