(Adds detail, analysis) DUBAI, June 30 (Reuters) - Net foreign assets at Saudi Arabia's central bank fell to 2.521 trillion riyals ($672.2 billion) in May, down by $6.6 billion or 1.0 percent from the previous month as the kingdom continued to draw down reserves to cover a budget gap caused by cheap oil. Deposits fell 22 percent between August and May, while securities holdings were just 7 percent lower.