Stagnation of activities can come easily, for example a facebook discussion stops as everyone is waitjng for everyone else to take action or to speak up.
The solution comes in two fold.
First, build momentum by engaging those who speak up. Don't worry about being inclusive during the discussions. What you want to do is bring the conversation to a good frequency, at which point hopefully the quiet ones will join in.
Second, be a "nagger". Always check in with the members. Are they going well with their part of the work? Any difficulties they may be facing? Deadline in six days? Touch base with them in three days. You may be naggy, but you are engaging.
