ISIS are quite strong actually because they have got millions in oil revenues and are making millions for other rackets like extortion and kidnapping. They also have some very rich backers in Saudi Arabia and Qatar. Plus an almost endless supply of recruits from across the world who are flocking to Syria to fight for them. They are also web geniuses using multiple twitter, Facebook and other social networking accounts to spread their propaganda and entice new recruits. Their ranks have also swelled massively to over 200,000, see this:
War with Isis: Islamic militants have army of 200,000, claims senior Kurdish leader - Middle East - World - The Independent