(Reuters) - Facebook Inc beat analysts' estimates for quarterly revenue on Wednesday, and said it has seen signs of stability in ad spending during the first three weeks of April. Shares of the company, which owns WhatsApp and Instagram, were up 9% in extended trading. Advertisers have pulled or cut marketing budgets to rein in costs because of virus-related uncertainty, including many of the small businesses and direct-to consumer brands that advertise heavily on Facebook.Facebook said..