public class SocialTokenGranter extends AbstractTokenGranter {

    private static final String GRANT_TYPE = "facebook_social";
    GiraffeUserDetailsService giraffeUserDetailsService; // custom UserDetails service

    SocialTokenGranter(
            GiraffeUserDetailsService giraffeUserDetailsService,
            AuthorizationServerTokenServices tokenServices,
            OAuth2RequestFactory defaultOauth2RequestFactory,
            ClientDetailsService clientDetailsService) {
        super(tokenServices, clientDetailsService, defaultOauth2RequestFactory, GRANT_TYPE);
        this.giraffeUserDetailsService = giraffeUserDetailsService;
    }

    @Override
    protected OAuth2Authentication getOAuth2Authentication(ClientDetails clientDetails, TokenRequest request) {

        // retrieve social token sent by the client
        Map<String, String> parameters = request.getRequestParameters();
        String socialToken = parameters.get("social_token");

        //validate social token and receive user information from external authentication server
        String url = "https://graph.facebook.com/me?access_token=" + socialToken;

        Authentication userAuth = null;
        try {
            ResponseEntity<FacebookUserInformation> response = new RestTemplate().getForEntity(url, FacebookUserInformation.class);

            if (response.getStatusCode().is4xxClientError()) throw new GiraffeException.InvalidOrExpiredSocialToken();

            FacebookUserInformation userInformation = response.getBody();
            GiraffeUserDetails giraffeSocialUserDetails = giraffeUserDetailsService.loadOrCreateSocialUser(userInformation.getId(), userInformation.getEmail(), User.SocialProvider.FACEBOOK);

            userAuth = new UsernamePasswordAuthenticationToken(giraffeSocialUserDetails, "N/A", giraffeSocialUserDetails.getAuthorities());
        } catch (GiraffeException.InvalidOrExpiredSocialToken | GiraffeException.UnableToValidateSocialUserInformation e) {
            // log the stacktrace
        }
        return new OAuth2Authentication(request.createOAuth2Request(clientDetails), userAuth);
    }

    private static class FacebookUserInformation {
        private String id;
        private String email;

        // getters, setters, constructor
    }
}

private TokenGranter tokenGranter(AuthorizationServerEndpointsConfigurer endpoints) {
        List<TokenGranter> granters = new ArrayList<>(Arrays.asList(endpoints.getTokenGranter()));
        granters.add(new SocialTokenGranter(giraffeUserDetailsService, endpoints.getTokenServices(), endpoints.getOAuth2RequestFactory(), endpoints.getClientDetailsService()));
        return new CompositeTokenGranter(granters);
    }