[activity stream] Activities for a user may show shared objects with another user

For some reason, objects shared with user X are also appearing to another user on the activity list. The case occurs with, for example, for user tpereira on object http://beatweb-staging/toolchains/chichan/full_pre_mlbphs_projection/1/

As you can see from the list, it looks like user chichan shared that object with user tpereira. When the user tpereira clicks on the contribution, he gets a 404 (which is correct, since the object was not shared with that user).