The steep declines we've seen this year have been awfully painful to folks who already own the stocks that have plunged. Luckily, times like these often create opportunities to start new positions or average down on the ones that fell. 

These stocks are more than 50% below the high-water marks they set not long ago. Scooping up shares at their recently reduced prices could go a long way to boost your overall portfolio in the years ahead.

PubMatic

PubMatic (PUBM -4.42%) is an up-and-coming player in the rapidly shifting advertising industry. Publishers and app developers use the sell-side platform to auction off their ad inventory to the highest bidders.

PubMatic's revenue mostly comes from revenue share agreements with its publishers, and they're obviously happy with the service. The company's net dollar-based retention rate during the 12 months ended March 31, 2022, was 140%. This means existing ad buyers are spending more on PubMatic's platform and its publishers are committing more of their inventory.

In the first quarter, PubMatic earned $17 million before subtracting interest, taxes, depreciation, and amortization (EBITDA). That was 17% more than the previous year's period, but this wasn't good enough for the market. The stock has fallen 77% from its peak last year. 

PubMatic sports an $843 billion market cap at recent prices, which is minuscule compared to its addressable market. According to eMarketer, U.S. advertisers spent $106 billion on programmatic display ads in 2021, and that was 41% more than they spent a year earlier. 

Global spending on advertising reached $772 billion in 2021, and this figure is expected to reach $1 trillion in 2026. PubMatic could grow to many more times its current size, as programmatic ad buying accounts for an increasing share of this already enormous market.

Duolingo

Duolingo (DUOL -2.63%) is an education company that specializes in languages. The company consistently crushes Wall Street estimates, but the stock is down about 57% from the peak it reached in 2021.

Duolingo operates the language learning application of the same name. The app had 2.9 million paid subscribers at the end of March, which was 60% more than it had a year earlier. All of those subscribers make it the highest-grossing education application on Apple's App Store and Alphabet's Google Play Store.

The Duolingo app employs a "freemium" model, and the vast majority of its 49.2 million monthly active users aren't paying. They are, however, providing the company with lots of data, so it knows which lessons to keep and which ones to prune.

Native English-speaking Americans typically use Duolingo to casually brush up on a language before traveling. In non-English-speaking countries, though, learners are more highly motivated by career opportunities. When the company went public last July, there was a single paid subscription price for every country. This year the company started adjusting its prices on a country-by-country basis, which could result in heaps more paid subscriptions from international customers.

Duolingo's also boosting its international footprint with an English proficiency test that is already accepted by Yale, Johns Hopkins, and more than 3,500 other institutions. In the first quarter, Duolingo reported $8 million in English test revenue, a 60% rise year over year, and there's clearly a lot of room for more growth in this niche. Duolingo's largest competitor for English proficiency testing, IDP Education of Australia, expects over $300 million in revenue from its proficiency test, called IELTS, this year. 

At the moment, shares of Duolingo trade at just 11.3 times trailing sales. That's an awfully low multiple for a company that expects its sales to grow by 56% this year. With an attractive price, a blazing fast growth rate, and a huge addressable market, this stock is a screaming buy right now.