It might not be related, but I've had troubles with this when using the social form fill. If a lead goes to your form, clicks on the social form fill button and logs in, but then abandons the form, it would still create the lead -- but not follow the lead flow of if they had completed the form.
I explain it better in this idea: https://community.marketo.com/MarketoIdeaDetail?id=08750000000JmxIAAS
I assume your issue is similar, in that clicking the sharing button pulls in what they can from the social network, but won't follow any flow actions unless you set up some edge case trigger campaigns.