Investors sitting on long-term gains in Big Tech names such as Facebook (FB) , Alphabet (GOOGL) and Adobe (ADBE) need to be careful about deciding to sell them, according to Jim Cramer.

"What you have to say is: 'OK what is Facebook going to earn in the out years?'" Cramer said in an exclusive conference call with members of his Action Alerts PLUS club for investors. "In 2019 I think it can earn more than eight bucks, maybe $8.50. I put a 20 multiple on that and what do I get? Well, I get a stock that is much higher than where it is now. That's the way I look at these long-term growth stocks."

While Adobe shares are high, according to Cramer, "I just spoke to [CEO] Shantanu Narayen and my conviction that Adobe's inexpensive in the out years is great."

Likewise, in the case of Google owner Alphabet, Cramer is very bullish. "I think that Alphabet is worth so much more and I would buy it right here."

But Cramer drew a sharp distinction between established names like Facebook, Google and Adobe and newly public Snap (SNAP) , which he said "is not going to make anyone any money" if they buy it at current levels.

But for the big names, "I can't get rid of these stocks just because they've moved up a lot, because the future is so great."

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At the time of publication, Action Alerts PLUS, which Cramer co-manages as a charitable trust, was long FB, GOOGL and ADBE.

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