Twitter Inc.’s user growth is slowing and it shows no sign of turning a profit. Some fund managers say that’s not going to stop the microblogging service’s $12.8 billion valuation from treading higher. Much higher. The $12.8 billion figure is derived from the fair value that Twitter put on its shares in an initial public offering filing last week. Ironfire Capital LLC and Gamco Investors Inc. (GBL) project the San Francisco-based company could be worth $15 billion to $20 billion once it begins trading.