Aug 6 (Reuters) - Shares of Robinhood Markets Inc (HOOD.O) rallied 10% on Friday after a roller-coaster week that has made the online brokerage among the hottest of so-called "meme" stock and added $18 billion to its value. Robinhood, whose smartphone trading app has taken off this year along with a frenzy of trading by retail investors on Reddit, was last at $56.20, up almost 50% from the price in its initial public offer last week. The company plans to announce its quarterly results on Aug. 18, Robinhood said in a press release on Friday. Robinhood also said it does not expect that it will be in a position "to obtain a declaration of effectiveness" from the Securities and Exchange Commission, which would clear the way for certain shareholders to sell their holdings, until after it reports its quarterly results. The stock stumbled in its first day on Wall Street before lifting off this week and surging as high as $85 on Wednesday, evocative of rallies in GameStop (GME.N) and AMC Entertainment (AMC.N) that Robinhood trading helped fuel earlier this year. At its market capitalization of over $47 billion, Robinhood is more valuable than companies, including Spotify Technology (SPOT.N) and Walgreens Boots Alliance (WBA.O). Retail investors made net purchases of $8 million worth of Robinhood shares on Thursday, according to Vanda Research, even as the stock tumbled 28% after a filing showed more than a dozen early investors could sell their stock over time. read more On Friday, Robinhood was by far the most discussed stock on Reddit"s wallstreetbets trading forum, with most comments reflecting positive sentiment, according to Swaggystocks.com, which tracks comments on Reddit. Robinhood was traded more than any other stock on Fidelity"s online brokerage, with buy orders outpacing sell orders by about 1.2 to 1, data on Fidelity"s website showed on Friday. Star investor Cathie Wood"s Ark Innovation ETF (ARKK.P) now owns Robinhood shares worth about $275 million after its stake increased by about 24 thousand shares on Thursday, according to data on the fund"s website.