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There's no question that online dating has fast become a booming space. The industry, which was nonexistent a generation ago, is now worth an estimated $2.5 billion in the U.S. alone, and counts tens of millions of customers around the world.

Match Group (MTCH) is the world's leading online dating service, but the industry remains highly fragmented (as the company's own portfolio of 45 brands indicates). Not only is Match Group facing dozens of competitors, but it also operates in a fast-moving space as customers turn over quickly and trends change with new technology. Tinder, for example, Match Group's popular mobile app and primary growth driver, only launched in 2012.

In order for investors to understand Match Group's prospects, it's important to understand the competitive landscape. Let's take a look few other online dating brands and companies that pose the biggest threat to Match Group's empire.

1. eHarmony 

Historically, eHarmony has probably been Match Group's biggest competitor. The questionnaire-based online dating platform brings in about $300 million a year, but tends to attract an older demographic than Match Group, which is increasingly focused on younger users through Tinder. 

According to industry observers, online dating is increasingly moving to app-based services like Tinder that are based on appearance or immediacy, rather than eHarmony, which takes more of a time commitment upfront. 

eHarmony is still a private company, as it has been since its launch in 2004. Since eHarmony is more of a competitor to, Match Group's legacy subscription service that focuses on an older demographic, and not Tinder, it will probably not be a significant threat going forward.

2. Spark Networks

Spark Networks, the parent of over 30 dating sites including ChristianMingle and JDate, is the only other publicly traded online dating company. However, its fortunes have been much less auspicious than Match Group's. Spark Network has been public for more than 10 years, but it is only worth $35 million today. 

In its most recent quarter, it made just $8.4 million in revenue and reported a loss of $94,000. Spark stock peaked at more than $8 in 2013, but has fallen steadily since then, in part due to the rise of Tinder, and today trades at just north of $1.

Spark went through a board break-up a few years ago, and the company has turned to swipe-based apps including JSwipe, but with little success.

Like eHarmony, Spark's potential as threat to Match Group seems to have faded in recent years.

3. Bumble

Spark Networks and eHarmony are both listed among Match Group's competitors along with several other veteran online dating services, but if a meaningful threat arises to Match, it may be more likely that it comes via a new app like Bumble.

Bumble was founded by former Tinder exec Whitney Wolfe, who sued Tinder for sexual harassment after she was forced out of the company. Bumble functions much like Tinder and similar apps -- users swipe right if they're interested in the match -- but with an important twist. On Bumble, only women can make the first move. Wolfe says that unique feature was designed to give women the ability to make the first move without seeming desperate or too forward. 

The result is that Bumble has become a much more female-centric dating app than Tinder, and it's taken off as a result. By July 2016, Bumble had approximately 7 million users, a fraction of Tinder's user base, but strong growth for an app that is less than two years old.

Tinder's growth continues to skyrocket as paid users have more than doubled in the last year, but apps like Bumble could one day present a serious threat to Tinder.

With revenue growth of around 20% this year, Match Group seems to be fending off its competition, but in this fast-changing industry, investors will want to keep their eye on potential challengers to Tinder and Match's other valuable brands.