Alisa,
Leads acquired by the following Social Apps are part of the "Socially Acquired" count -
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Social Button
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Youtube video
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Poll
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Referrals
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Sweepstakes
Even though the leads did not register, it looks like they shared with the social button and during that process authenticated with a social network. When they do so, we create a new lead in the system and set Origial Source Type to "Social App".
If you want to track the number of registrations through social shares, you should consider using the referrals or sweeps app. These apps allow the users to share and also help you track how many registrations each of your referrers bring in.
Hope this helps,
Raj