SSRIs tend to have a rather broad binding affinity. Most of them bind not only to serotonin transporters, but also norepinephrine, dopamine, and 2,700 other things to a lesser degree. Especially curious are factors like agonism of the poorly-understood sigma receptors, which now seem heavily implicated in depression, and also the fact that many SSRIs have been demonstrated to affect NGF and/or BDNF, which modulate plasticity. So the action on serotonin may well be completely irrelevant. Since depression is a disorder of the default mode network, wherein (essentially) the channels of communication become so well established that it is very difficult to break free, there is a whole lot of chatter in the research community about how we are all but convinced now that the neuroplastic effects are more responsible for improving depression symptoms than anything to do with serotonin. But there is also a very compelling reason to believe sigma receptors might be involved, because many drugs known to work as rapid antidepressants share sigma agonism in common (and very little/nothing else).