Jones et al. (2008) (don't you love academic referencing?) found empirical evidence that information overload significantly constrained interaction between members.
They discovered that 40 participants interacting within 20 minutes was the maximum number which could be sustained.
As the volume of messages increases. Users are:
1) More likely to respond to simpler messages (shorter, dumber, fun).
2) More likely to end active participation.
3) More likely to generate simpler responses.
40 participants might not be the exact number, especially within forum platforms.
But there is a number...and that number is very important.
Beyond that number the quality of interaction plummets, the number of active members plummets (in favour of fewer, highly active members, posting silly comments to each other).
The challenge is to identify that number and take steps to prevent information overload. Developing sub-groups of friends or interests would help. You can split one popular category of discussion into several and reduce the danger of information overload. Dissipating activity is your best weapon.
You might also tighten the moderation policy, disallow dumb comments, increase the quality of discussion taking place.
You can also turn a major topic of discussion into a weekly live chat for interested members. This might boost activity without decreasing membership. We recommended this to WarriorCats a few weeks ago. Likewise, you might add a chat room to a forum, or a forum to a chat room. Now you can have a relaxed policy in the chat room and a stricter policy in the forum.
Your mileage with any of these routes will vary. What's important is the number. It exists. It might be 40 participants in 20 minutes, it might be 100 in 10. Make sure you identify your number and prevent information overload.