I am using metadata API to retrieve the LastModifiedBy of an Object. But, it returns me only one User.
I wanted to know if we can track, the Users whoever modified the Object. Is it possible by any means?
I think you're looking for the Setup Audit Trail. This setup screen should display the last 20 metadata modifications, the last 6 months can be downloaded as csv.
Update: Apparently this data is not exposed/supported by the Metadata API (or any other). There is an idea on the IdeaExchange on which you can vote: Make Audit Setup Trail History available via the API
Yes we can track all changes to salesforce through Audit Trial. There are some tools available in appexchange like AutoRABIT where we can track changes to all metadata memebers with given date and time. By using advanced search options we can search by date,metadata type,modified user,component name etc.