Some of the world's biggest technology companies stumbled through 2015, while others sprinted. Now that the year's coming to a close, there's a handful of mobile headlines that helped define 2015. 

Let's a take a quick look at five of the biggest mobile stories of 2015:

1. Microsoft writes down its Nokia purchase
Over the summer, Microsoft (MSFT -0.32%) announced it would write down $7.6 billon from the devices and services division it bought from Nokia, which easily surpasses the $7.2 billion Microsoft paid for Nokia's mobile division. 

Microsoft worked with Nokia to produce Windows smartphones for years before deciding that taking over Nokia's devices and services would boost Microsoft's position in the smartphone space. Aside from the writedown, Microsoft also cut 7,800 jobs, with most of them related to the Nokia purchase. 

Research firm IDC had this to say about the failed Nokia purchase: "Since its acquisition of Nokia in 2014, Microsoft has been revamping the product portfolio with Microsoft branded Lumia devices. But now that Microsoft has decided to take a loss on its Nokia purchase, the scenario for Windows Phone looks bleaker." So much for boosting Windows smartphones. 

Microsoft's inability to create something good out of the Nokia purchase forced me to name Windows phones the worst Microsoft product of 2015. There's always next year, Microsoft. 

2. Apple has its best iPhone launch to date
Apple (AAPL -1.00%) stepped into the wearables market this year with the Apple Watch, but it was iPhone sales that stole the show. The iPhone 6s was the most successful Apple phone launch to date, selling 13 million units in the first three days. 

While that was an all-time record for Apple, those sales numbers come with a small asterisk next to them. This year's iPhone launched in China at the same time it hit the U.S., which helped boost the initial launch weekend sales numbers higher than those of the previous year's iPhone 6 (which didn't become available in China until a month after the U.S. debut). But there was more good news for Apple earlier in the year as well. Apple had its second-highest quarterly iPhone sales ever, topping 61 million units in the fiscal second quarter.

As if that weren't enough, we found out this year that Apple takes 92% of all smartphone profits worldwide, up from 80% in 2014. 

3. Android's security problems come to light
There are currently lots of different versions of Android out there, made by different companies, updated on separate schedules or simply never updated at all. This has made the responsibility of securing Android phones a bit murky at best. And all of that helped lead to the Stagefright bug, which became a massive security problem for devices built on  Alphabet's (GOOG -0.02%) (GOOGL -0.09%) Android.

Stagefright had the potential to infect 1 billion Android devices, and could gain access to a device with just a single, untraceable text message.

"These vulnerabilities are extremely dangerous because they do not require that the victim take any action to be exploited. Unlike spear-phishing, where the victim needs to open a PDF file or a link sent by the attacker, this vulnerability can be triggered while you sleep," Zimperium Mobile Security said in a blog post this year.

The vulnerability led Google to start issuing its own monthly security updates for the latest versions of its Nexus phones. And device makers like Samsung and LG also committed to issuing more regular security updates for their devices as well. 4. Samsung's mobile profits hit the skids
(NASDAQOTH: SSNLF) is the world's largest smartphone maker, but this year the company's mobile profits took a hit. Just two years ago Samsung's mobile division made about about 66% of the company's total revenue. This year that number fell to just 32%. 

Part of the problem has come from lower average selling prices (ASPs) for the company's devices, which have come from increasing competition from other Android makers. Samsung is having a hard time differentiating itself from the rest of the Android world, and so far it doesn't seem to have an answer to this problem. And while the Galaxy S6 and S6 Edge weren't exactly flops, Samsung failed to meet the strong demand for the S6 Edge, and was left with too many unsold S6 models. 

As Apple's iPhones have dominated smartphone profits in 2015, Samsung has been left trying to put back together the broken pieces of its once lucrative mobile division.

5. Activision Blizzard buys Candy Crush maker 
Toward the end of 2015, we saw Activision Blizzard (ATVI) fork over $5.9 billion for mobile gaming company King Digital (KING.DL). Activision is known for its Call of Duty and World of Warcraft games on PCs and consoles, but scooping up the Candy Crush maker gives Activision a foothold in the mobile gaming space. According to Activision CEO Bobby Kotick: 

The combined revenues and profits solidify our position as the largest, most profitable stand-alone company in interactive entertainment. With a combined global network of more than half a billion monthly active users, our potential to reach audiences around the world on the device of their choosing enables us to deliver great games to even bigger audiences than ever before. 

As fellow Fool Tim Beyers wrote last month, King could bring in 30% of Activision Blizzard's fiscal 2016 revenue and earnings, and the combined non-GAAP revenues from both companies equaled $6.8 billion over the past 12 months. The purchase comes at a great time, with total revenue from the mobile gaming market hitting $36 billion this year and expected to increase by 50% over the next four years.

While Apple and Activision Blizzard made out the best this year, I wouldn't be surprised to see Microsoft make some additional headway in 2016, and for Samsung to reposition itself to increase mobile profits.