Several Wall Street firms are cautioning their clients about the prospects for Twitter shares, saying the social media company's stock will not outperform until user growth returns.

On Friday the company posted slightly lower-than-expected second-quarter monthly active user (MAU) numbers. But the bigger disappointment was Twitter's guidance for the third quarter, when it forecasts a decline of "mid-single-digit millions" in monthly users from the second quarter.

Twitter shares fell 20.5 percent on Friday after the company earnings report. The stock fell another 8 percent Monday, resulting in a nearly 27 percent loss over two days.

Bank of America Merrill Lynch reiterated its underperform rating for Twitter shares, citing the company's forecast for a decline in users.

"Slight 2Q beat [is] overshadowed by outlook for slowing growth," analyst Justin Post said in a note to clients Monday. The "outlook suggests revenue growth rates may have peaked, monthly users could decline, and platform health initiatives will impact margins, and we would expect less ongoing optimism for continued financial upside in the stock."

Post reiterated his $27 price target for Twitter shares, representing 21 percent downside to Friday's close.

One Wall Street analyst said investors should avoid Twitter shares until user growth returns.

"We believe upside to current levels would require growth in the overall user base, which we see no evidence of at this point," KeyBanc Capital Markets analyst Andy Hargreaves said in a note to clients Friday.

Hargreaves reaffirmed his sector weight rating for Twitter shares and said his "fair value" for the stock is $32.

Stifel praised Twitter management for its initiative to clean up the platform but also doesn't see much upside for its shares.

"With everything going on in the world (and with Twitter's peers), it's hard to fault Twitter for prioritizing the long-term health and viability of its platform for public conversation, but it's even more difficult to justify why investors should own the stock as it goes through this period," Stifel analyst John Egbert said in a note to clients Friday.

Egbert reiterated his hold rating and raised his price target to $30 from $27 for Twitter shares.

To be sure, not every analyst is giving up on Twitter's stock.

J.P. Morgan analyst Doug Anmuth told his clients Monday to "buy the selloff" in Twitter shares.

"Slightly light 2Q results and the below-consensus 3Q outlook were clearly disappointing, and there is somewhat of a reset on numbers coming out of the quarter. However, we do not believe that Twitter's underlying fundamentals have changed," he said.

Anmuth reiterated his overweight rating for Twitter shares and lowered his price target to $45 from $50 for the company's stock.

Twitter did not immediately respond to a request for comment.