The date value in this field marks the first day a person has been tagged with a Marketo cookie by making a visit to the company's website. It can be filled that one time only. From that moment on, any subsequent visit made by the same person using the same browser & cookie, will be marked as an activity on that record's activity log, yet they may either remain anonymous in perpetuity, or until the day they become known.
Note: earlier this year, Marketo has deprecated the ability to use the 'Is Anonymous' filter and select unknown records from the database.
Lead was Created
The date value in this field marks the first day that a lead has been created, or has become known, in the database. This could be the result of that person filling out a form on the web, having been added via a list import, or through an API call, or via a sync action from the connected CRM. As of that moment, it is considered to be a Lead in the system. This field, just like the “Created At” field, can be filled once only.
SFDC Created Date
As its name implies, the value in this field marks the first date when a record has been created in Salesforce, regardless of its source.
Today is 07/04/2016. A person visits the webpage of Converto , reads some material, moving from one page to another, and downloading some reading material. In the meantime, since the person is not known in the system, all the information about the person is being saved anonymously, as it is being tracked by a cookie that was set in the person’s computer(similarly to the way their visits are tracked by Google Analytics). The person is now saved in Marketo, having ID and activity log, and the “Created At” field, filled with 07/04/2016.
A day later, the same person visits the same webpage and fills out a form. Once that happens, the credentials of that person are being linked to that ID that was given a day before. Now, the person has and ID, credentials, and “Lead was Created” field, filled with 07/05/2016.
Let's say that at Converto we don't sync the lead directly to our CRM, but instead, good B2B marketers as we are, we nurture new leads until they've provided the required signals telling us they're 'hot' enough to talk to a sales person. Let's also say this happened two weeks later. So at that point the field "SFDC Created Date" is populated with the date 07/19/2016.