Investing.com - It took more than a month, but a Wall Street firm has downgraded Facebook (NASDAQ:FB) in the wake of its data privacy scandal.OTR Global lowered its rating from positive to mixed, forecasting lower year-over-year growth in advertising spending . The research firm expects growth in ad spending to be 19%-24% higher than a year ago, compared to a 30% gain in the fourth quarter.OTR's is the first downgrade, although several other firms have lowered their stock-price targets for Facebook since the social media giant revealed that Cambridge Analytica had misused the personal data of millions of users.Facebook shares, which had lost as much as 17% of their value in the wake of the scandal, are down 13% from their February high.The company reports earnings on April 25.