* Facebook cites trading glitch, oral disclosure reports * Defends "customary practices" on Wall Street * Market value has fallen by a quarter since debut * SEC warned Facebook on mobile - new filings By Basil Katz and Edwin Chan NEW YORK/SAN FRANCISCO, June 15 (Reuters) - Facebook Inc , facing a raft of lawsuits from investors seeking to recoup losses from its botched IPO, laid out on Friday how cascading Nasdaq trading glitches might have stoked the confusion that marred