It's been the biggest year for M&As in history, according to the financial software company Dealogic. But there wasn't a massive, What'sApp-size buy this year.

Inc. looked at startups less than 10 years old, and privately owned, that did manage to score very significant deals in 2015--even if they weren't quite in the multibillion-dollar range.

This year's headliners, which accomplished the biggest exits--of more than $100 million, and with figures that have been published by reliable sources--may surprise you.

20. Twitter acquires Periscope.

Date: March 9, 2015

Price: Less than but up to $100 million, according to reports

Early bird: Notably, Periscope--an app for streaming video from a cell phone--was acquired before it even launched. In fact, the company was still working on applying to Apple's App Store. (The pressure was on because a competitor, Meerkat, had recently launched and seemed like it was gaining user momentum.)

17. (Tie) Dropbox acquires Clementine.

Date: July 22, 2015

Price: Estimated to be $100 million

Baby buy: This startup, which focuses on internal communication, such as conference calls and chat services not connected to a personal phone number, was less than one year into offering its product when this acquisition went down.

17. (Tie) Dropbox acquires CloudOn.

Date: January 21, 2015

Price: Approximately $100 million

Location, location: The Israeli company that provided mobile applications for document editing and creation had nine million users--and its 30-person team now forms Dropbox's first office in Israel.

17. (Tie) Microsoft acquires Sunrise Atelier.

Date: February 4, 2015

Price: $100 million

Easy does it: Sunrise, the maker of a suite of calendar apps and programs founded in 2012, may be part of Microsoft's new bid to be hip, and have a friendly interface for users. In an unusually casual move, it announced the acquisition via YouTube video.

16. Raycom Media acquires PureCars.

Date: October 27, 2015

Price: $125 million

Band of outsiders: This digital-ad firm isn't in Silicon Valley or Alley--it's from Charleston, South Carolina. And its 140 employees attained rocket revenue growth--1,401 percent over the past three years--by being a preferred partner of Google in the automotive ad sector.

14. (Tie) Infosys acquires Panaya.

Date: March, 2015

Price: $200 million

Global deal: One of India's largest software exporters, Infosys, acquired the enterprise-resource-planning startup Panaya, which was founded in 2006 in Israel and had raised nearly $60 million in funding.

14. (Tie) Yahoo acquires Polyvore.

Date: July 31, 2015

Price: Roughly $200 million

Sweet suite: The visual search engine for clothing, Polyvore, had nine million visitors to its websites and apps in June--far more than its competitors, such as Wanelo and Fancy.

13. Samsung Electronics acquires LoopPay.

Date: February 18, 2015

Price: $250 million, as reported by Re/code

Tech magic: LoopPay's technology turns magnetic-stripe readers into a new type of tap-like payment system, requiring no contact with a card. And it works with 90 percent of existing point-of-sale terminals.

12. Northwestern Mutual acquires LearnVest.

Date: March 25, 2015

Price: More than $250 million, according to Fortune

Getting personal: After a courtship involving a significant earlier investment, Northwestern Mutual bet big on personal financial planning by buying LearnVest, which was founded by business-school dropout Alexa Von Tobel.

11. PayPal acquires Paydiant.

Date: April 3, 2015

Price: Around $280 million, according to reports

Packing the bags: Boston-based Paydiant powers payment apps for large businesses, such as Subway. PayPal purchased it in the run-up to its separation from eBay this year.

10. Amazon acquires Elemental Technologies.

Date: October 19, 2015

Price: $296 million

Bidding war: The Oregonian notes that Amazon outbid several other tech giants, including Cisco, Ericsson, and Verizon for Portland-born Elemental, which creates high-speed video-delivery systems and employs around 250 people.

9. Microsoft acquires Adallom.

Date: July 19, 2015

Price: $320 million

Name game: The cloud-access security provider, founded in 2012, derived it's name from the Hebrew saying "Ad Halom," which literally means "up to here" and in game theory lingo means "the last line of defense."

8. AxelSpringer acquires Business Insider.

Date: September 29, 2015

Price: $343 million

Benchmark deal: The culmination of a long courtship, the sale breathes new wind into the New York City media upstart ecosystem, and shows old media's faith in the value of tech-infused online publications.

7. Amazon acquires Annapurna Labs.

Date: January 22, 2015

Price: $370 million

Secrets, secrets: The Israeli chip-maker didn't even note the sale at the time--and little was known about it then. Amazon's cloud-computing unit took the company, named for a Himalayan peak, into its division.

5. (Tie) NeuStar acquires MarketShare.

Price: $450 million

Date: November 5, 2015

Bigger plans: The huge acquisition of this 10-year-old marketing-analysis startup will help ad-tech giant NeuStar give its clients more detailed strategies for ad buys, across media and in real-time.

5. (Tie) Pandora acquires TicketFly.

Date: October 7, 2015

Price: $450 million, reportedly

Founder chops: Back in 1995, TicketFly founder Andrew Dreskin created TicketWeb, the first online ticket-buying portal.

4. Hitachi Data Systems acquires Pentaho.

Date: February 10, 2015

Price: Between $500 and $600 million, according to Bloomberg BusinessWeek.

Analyze this: Pentaho was founded in Florida by five guys in 2005, and over the years raised about $70 million to create an open-source business intelligence, data integration, and analytics company. In essence, this was a big-data acquisition.

3. Cisco acquires Acano.

Date: November 20, 2015

Price: $700 million

All together, now: The London-based tech firm makes collaboration and conferencing software. In the age of Slack, collaboration and in-office communication is an in-demand area. And this company addresses scalability, allowing a business to connect tens of thousands of individuals at once.                             

2. Adobe acquires Fotolia.

Date: January 28, 2015

Price: $800 million

Living on: The decade-old stock-photo site had some 34 million images. Adobe is continuing to run the business, plus offer its services to its subscribers as well.

1. EMC acquires Virtustream.

Date: July 9, 2015

Price: $1.2 billion

Strategy shift: Virtustream, a cloud-software and -service provider founded in 2009 in Bethesda, Maryland, was preparing to go public for about a year before computer-data storage heavy-hitter EMC stepped in. (This, of course, was before Dell bought EMC for a whopping $67 billion in October--around the time EMC spun-out Virtustream as a separate entity.)