The Nasdaq 100, a market index heavily weighted with tech stocks, is getting very pricey. The index trades at a P/E ratio close to 23, well above the S&P 500's P/E of 20, highlighting the general market optimism toward some of tech's big names recently. With tech stocks getting pricier and valuations looking less enticing, investor watchlists may be getting smaller. On this note, here are three tech stocks that -- even in a pricey market -- are still interesting: Apple (NASDAQ:AAPL), Twitter (NYSE:TWTR), and LinkedIn (NYSE:LNKD).

Apple: Still cheap
Despite Apple stock's recent run-up, with shares climbing 64% in the past 12 months, it's trading conservatively in relation to its underlying earnings power and the company's growth potential.

Apple Watch Fashion

Apple Watch. Image source: Apple.

Sure, a $700 billion plus market capitalization is hard to fathom, but so is the company's $44.5 billion in net income in the past 12 months.

And, going forward, three key catalysts for the stock make its 16.4 price-to-earnings ratio look downright cheap.

  1. An aggressive share repurchase program.
  2. Apple's foray into wearables with the Apple Watch.
  3. A monstrous iPhone business that is firing on all cylinders.

Twitter: Mind-boggling monetization
In contrast to Apple, social-media company Twitter doesn't appear to be cheap at all -- at least when measured by traditional valuation metrics. But the underlying business is growing incredibly fast. Pair this growth with a strong network effect built around a very unique base of users, and Twitter stock is interesting in spite of the lofty premium the market is paying for growth potential.

The best way to show just how valuable Twitter's user base is is to look at the company's growth in advertising revenue per thousand timeline views, or ad ARPU. In Twitters most recent quarter, ad ARPU was up 60% from the year-ago quarter. The growth in this key metric was the primary reason behind Twitter's 97% year-over-year revenue growth in this same period. Monetization is proving to be no problem for this young public company.

Twitter Ad Arpu Q

Chart source: Twitter.

LinkedIn: A growth story for the long haul
Like Twitter, LinkedIn also appears to be a pricey stock when measured by traditional valuation metrics. Further, investors who didn't buy shares during the stock's pullback from over $250 in 2013 to $150 in 2014 may feel discouraged to even give the stock a closer look; today, shares are trading around $270.

Linkedin Member Growth Q

Monthly average members during the quarter. Chart source: LinkedIn.

But, also like Twitter, this company is growing rapidly. Last year's revenue was $2.22 billion, up 45% from the $1.53 billion it reported in 2013. The company expects revenue for 2015 to be between $2.93 billion and $2.95 billion.

Standing out from the catalysts likely to propel meaningful revenue growth from here is one unique opportunity that social companies Twitter and Facebook don't share: China. Since the company launched a localized version of LinkedIn in February, the company has doubled its Chinese member base from 4 million to more than 8 million.

Most important of all for long-term investors, LinkedIn's network of users make up arguably one of the world's most powerful network effects. Given the company's specialization in a professional network for knowledge workers and its dominance in the U.S., it wouldn't be easy for a competitor to stand up against the company. Best of all, at 93 million monthly unique visiting members, the company's network is big enough to fend off competition yet small enough that there's still plenty of growth ahead.

Is Apple, Twitter, or LinkedIn on your watchlist?

Daniel Sparks owns shares of Apple. The Motley Fool recommends Apple, Facebook, LinkedIn, and Twitter. The Motley Fool owns shares of Apple, Facebook, LinkedIn, and Twitter. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.