You may notice that some of your social network contacts, like Twitter friends, are missing in My Contacts.
This is because FullContact's Sync Settings for social networks doesn't allow automatically adding all Twitter or all Foursquare contacts to your synced contacts - this is primarily to avoid unwanted empty contacts (by unwanted, empty contacts, we mean contacts containing no other information besides a Twitter handle or Foursquare profile).
If you turn on 'sync' for social networks, this means that if the system can detect that there is a matching contact in My Contacts (if the names match + there are high-confidence identifiers in common), they will be automatically merged and the main contact entry will be updated with changes in the social network.
The main reason for doing this is so we do not sync contacts without any useful information (such as email addresses, phone numbers) to your Google, iCloud, or Exchange Contacts.
*However, we do recognize that this isn't for everyone, and you may prefer to have all of your Twitter, Foursquare, and LinkedIn contacts added to your synced contacts (i.e your My Contacts). In that case, you will have to add them to synced contacts one-by-one.