Yes you should be using RESTful service in order to create cross platform service. this way you can access your wcf service methods using
http://yourdomain.com//service.svc/users/{username}/bookmarks?tag={tag}
But there is a problem with consuming restful service you need to implement authentication in order to secure them properly to avoid DOS attack and other malicious use. Best approach to secure restful service is to use Hashing paramters in every service call. hash param could be derived from a secretkey,datetime, message salt etc.. for more info check this.
WCF, RESTful Web Services and custom authentication
I would suggest you should be creating restful service in JSON format so that it will be light weight and cause less overhead on data package.
http://www.codeproject.com/Articles/327420/WCF-REST-Service-with-JSON
Regards.