So, you want to know how automerging works. AFAIR, basically it's done by contact name only - so the local contact's name must match their name on the service (Twitter/Facebook).

I always thought that this should be a good deal smarter than it is, but UI design at the time dictated otherwise - that it should only create links based on obviously visible data. One day I'd really like to try prototype something that mashes together data from multiple services as smartly as it can, but I digress.