LONDON (Reuters) - Bitcoin hasn't been the only casualty of the backlash by the world's major economic powers against Facebook's plans for a cryptocurrency, with smaller digital coins also feeling the burn. Bitcoin has slumped around 30% from 18-month highs of nearly $14,000 touched after Facebook's move, following a growing chorus of concern among regulators and politicians from the United States to Europe at the social media giant's plans. And the so-called altcoins have fared even worse. On Wednesday, G7 finance chiefs cast a cloud over prospects for Facebook's Libra digital coin, insisting tough regulatory problems would have to be worked out first. After Facebook unveiled its Libra cryptocurrency, bitcoin soared as much as 55% in just nine days as investors bet the social media giant's gambit would herald mass adoption of cryptocurrencies.