There is a lot of discussion about this there;
Essentially with a trust level system any person can gain trust level;
Also any person can reduce their trust level through Network abuse therefore any person’s incentive is to maintain their trust level at a constant place.
An average human can keep track of 150 people in their field. Therefore; if a person say has 5000 “friends” those “friends” are getting spammed, because that one person does not at all know each and every 5000 people there.
In the system where the person has control over who is allowed to input into their stream can rank any person as intrusive. Thus this interaction will be blocked and clog the space of the spammer:
An interesting way to rank content; for example imagine:
A project site;
each project has several essential skills.
If those skills correspond to someone, that person can rank the project within their list. Others who rank the project in their list will see others who have ranked that project. Therefore the trust level of these people increases for eachother and messaging could be enabled for those only who have the trust level of ranking that specific project.
These people there can then individually select eachother to rank on their list. And once they are done suppose there are no common interests among those people; once the project is done, they can move on to their own lives.
The idea of free space should be tested; perhaps a certain network trust level could afford someone a free space; or into the future there will be an assessment to free space;
However, empirical evidence from such services at gmail and dropbox - these require a centralized anti spam mechanism. Though with decentralization automation with time I think we will see much improvement to these systems permitting spam less storage, etc.