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Trust is very difficult to define as it's too personal thing.
As people before me mentioned in online communities commitment is all too often mistaken for trust. For example taking computer games as an example: take a player that is extremely committed to the game, is always on time for events and possibly organises a lot of them. In this case I would agree that there may be someone else, who invests just as much time as him, that really trusts him, but could he be trusted by someone who neglects the game, or simply does not have time? Maybe, but not necessary.
So in my opinion would be that trust exist between friends and can only be (dis)proven by unexpected situations, that require some serious effort by one party without expecting any reward.
In various projects/games/etc what you want is commitment, and if you can befriend and trust someone - it's a bonus.
As for collaboration one way to do it (naturally that would possibly require some modifications to system) would be to disperse different messages to different users, not unique but just a few variations, and make users find who has the other part they need, or simply make them do something in a group and add a way to anonymously vote contribution by other members of a group.