The salesforce marketing cloud connector recommended for a long time to use the email address as subscriber key. With the newest release it is possible to use data streams or data sources to connect salesforce and exactTarget. It is possible to create a synchronised data extension called Contact__Salesforce. The subscriber key that is used is the contact id.
- How can I do the transition between Email and ID?
- Is it possible to track Emails in Salesforce with the new data source technology?