In cases where a party experiences difficulties in serving the claim form or other documents, the court has power to allow service by an alternative method or at an alternative place (CPR 6.15 and CPR 6.27).
In this case, the solicitor concerned apparently attempted to serve the defendant using conventional methods without success and persuaded the court that service via Facebook would be an appropriate alternative, since the defendant visited the site frequently. For more information on alternative service, see Practice note, Service: asking the court for help: Court's power to allow alternative service.
This development is further indication that the English courts are willing to take a modern approach to service by adapting procedures to new technologies.