3 big tests for Facebook at F8

SAN FRANCISCO – Facebook has a long list of challenges to address Tuesday when it kicks off its annual gathering for developers and partners to see the latest changes to its arsenal of hit apps. By most measures the company is almost invincible – each month at least 1.59 billion people use Facebook, more than a billion people use WhatsApp, 900 million use Messenger and 400 million are on Instagram. But the bigger Facebook becomes, the more it has to tackle nagging symptoms that could