I think our attempt had some issues... we were loudest near the end when the game was more or less in hand, and the action (Marshawn stepping over the goal line, untouched) was at the opposite end of the field (north end) from where they were monitoring (south end). As noted by others, there was no support or timing suggestions from the Seahawks and certainly no efforts to encourage fans to roll up programs to make megaphones as there was in KC.
I can also vouch that I've been to at least three games where it was louder, substantially louder than the 49ers game. NFC Championship 2005, the Giants 11 false starts game (these things have a tendency to feed into each other, when you start getting the false starts, the crowd gets louder), and Beast Quake. Hell, I think our previous 49er game at home was louder than this one.
Long story short, I think we can easily take the record back, if we want to. Schedule another record attempt, sync it up with a rival, get people fired up for the attempt, even consider handing out "signs" that can be rolled up into megaphones, have the scoreboard recommend loudness during a defensive series, and we'll have it again.