Facebook Stock RatingUPDATE, 3:00 PM: The stock closed at $21.71, -6.2%, a new low. During the day shares traded for as little as $21.61, also a low for Facebook. Tuesday was the stock’s 12th most active trading day, with 55M shares changing hands. The company has lost 26% of its market value since Thursday, when it reported Q2 earnings.

PREVIOUS, 8:44 AM: The stock is down more than 5% to less than $22 at mid-day after Bernstein Research’s influential analyst Carlos Kirjner lowered his target price to $23 from $25 — and upgraded Facebook to “market perform” from “underperform.” The stock, which went public in May at $38, has already set a new intra-day low of $21.82. Facebook’s just worth about $19 a share based on its display ad business, Kirjner says. New opportunities, including ad sales for mobile users, remain “highly uncertain” — justifying only an additional $4. The company’s prospects are “very much tied to its ability to monetize social advertising on mobile devices, something it has not yet proven it can do at scale and on a sustained basis.” His analysis notes that since so many adults in North America and Europe already use the social network site, “it is hard to see how new users will drive growth in these regions (we find the idea that Facebook could grow its user base by targeting kids fraught with risk and downside for the company).”

Related: Facebook Shares Hit New Low Following 11.7% Drop

He’s also concerned about Facebook’s disclosure, with its Q2 earnings, that ad rates declined in Europe. That suggests either that the return on investment for Facebook advertisers “is just not that attractive, or the company has a long and arduous path to making it transparent to advertisers,” Kirjner says. Facebook has lost nearly 25% of its value since Thursday when it reported its quarterly results.