My company specializes in user and customer acquisition for mobile and web based products.
It's important to note that:
1) Early-stages startups are not interested in such services because: they don't have money for it / it's too early to scale anything until traction.
2) The other thing - more mature startups: seed, Series A and later.
It's right audience. Because as soon as you prove your hypothesis and get a traction, it's time to scale it.
But in this case it's better to formulate a question:
"What kind of marketing channels I need to use?". It's a very wide and abstract question.
But usually it works like:
1) Direct Sales (confs, linkedin);
2) Content Marketing (Good guestposts);
3) Partners program.
In reality there is no big problem. Any startup which crossed "traction line" has a need to leverage more quality traffic. And I don't think that it's a problem to reach them.
The main problem to provide them with really good user acquisition in terms of Price / Quality. It's a hardest part.
There are thousands companies who do the same. In my taste, only 1% provides service which worths of attention.