Twitter (TWTR) fell in post-market trading Thursday after RBC Capital downgraded the stock to Underperform, saying the firm's recent advertiser survey shows waning interest in the platform.

INTEREST TURNING NEGATIVE: In a research note issued this afternoon, RBC analyst Mark Mahaney downgraded Twitter stock to Underperform while cutting his price target to $14 from $17. The research firm's survey of more than 1,100 advertising professionals and their attitudes towards Facebook (FB), Google (GOOG), Twitter and other high-profile names showed that, despite marketing budgets continuing to shift online, interest in the microblog platform appears to be declining. Notably, just 26% of respondents indicated plans to boost Twitter spend while 28% said they plan to decrease, which is "the weakest result we have seen and the first time we have seen a negative skew towards spending," according to Mahaney. Thirty percent of survey takers were not allocating any funds to Twitter, up from 25% in February. Only 24% said they believe their Twitter ROI has improved versus 21% who believed it has declined, and the platform was ranked fifth out of seven in terms of ROI to advertisers, ahead of only Yahoo (YHOO) and Verizon's (VZ) AOL. All that said, the analyst noted that Twitter remains a "unique asset with a strong value proposition to core users."

PRICE ACTION: Shares of Twitter are down 3.27% to 18.02 in after-hours trading.