You just want to use DFP, MoPub, or AdZerk, I suspect. There are others - they will probably answer this question as part of their marketing activities. But DFP has basically all of the market share.

Let's be clear, maximizing ROI takes work. Great ROI is a combination of optimization of advertisers (e.g. network/exchange mix, floor strategies, etc.) and inventory mix. For example, demand for interstitials and video is high, hence CPMs are higher than banners, which are somewhat commoditized. Location data is valued. Carrier and OS combinations are valued. I am assuming you are ok with decent ROI and are seeking a blend of ROI and convenience. Convenience typically means MoPub or AdMob (Google). Nexage, Smaato, and Millennial are other popular exchanges/networks in the mobile space. People like Pubmatic and Rubicon are slowly dipping their toes into mobile but are more effective today at monetizing mobile web inventory than app inventory (although they love app inventory as it typically has better performance characteristics).

My professional opinion, powered by research: - In 2011, Pacific Crest surveyed SaaS businesses about churn rates and more than 75% had an annual churn rate below 5%.[1] - If you are targeting SMBs, conventional wisdom is an annual churn rate of 15% is good (1.17% monthly)[2].  [1] http://www.pacificcrest-news.com/saas/Pacific%20Crest%202011%20SaaS%20Workshop.pdf [2] http://blog.hopkins.io/2014/03/20/churn/

If 30% of the people that come to page paypal you money, that is probably pretty good.

I would pick up the phone. I think actually speaking with the decision-maker will illuminate how to reach them at scale, objections, and other things you need to know.

This depends on what you are doing. Let's start with some caveats: 1) Speed is important - you don't want to spend a lot of time screwing around, you just need some traffic 2) Money is important - you would rather have some people on the cheap than a very well qualified audience that you paid more for. 3) You are pretty early - you don't have some massive social media following and stuff like that. Frankly, if you had that you would already be getting 50 users per day. Assuming these caveats are true, people suggesting things like Google/social media/etc. are probably heading you in the wrong direction. You probably don't want to spend days and days hunting around Google for pockets of low CPC keywords that have enough volume to get you the traffic you need. You probably want to just dial it up, set it and forget it. If #2 is not true, then Google Adwords is solid. You will pay a pretty high CPC, but you will get a very well qualified, bottom-of-the-funnel kind of audience. If both caveats are true, I would concur with other people that mention Facebook. Big audience + Low CPCs = easy to get 50 people per day. Things like "advertise on some podcast", "start a blog" or "get some press" seem like advertising mechanisms where you cannot control the amount of traffic you generate on any given day, may be below your target or above your max, result in spikey traffic, and may take a long time to actually get started. I wouldn't do that for testing. Now, I have no idea what you are doing, so the true, true answer may have some variability based on that, but these general guidelines are pretty good.

Explain the idea on Clarity and ask for feedback.

For prototyping, I think the best tools facilitate interaction and describe functionality without being overly high fidelity or requiring any design skill. (This is particularly important for me!) I find sketch interesting, and mirror is very cool, but it is more of a tool for designers, IM (very limited) HO. Similarly, PhoneGap and Appcelerator are for something else: actually building something. The code-ish nature of those products is the exact opposite of building a lightweight prototype. I have always used Omnigraffle to build prototypes both for mobile and the web. I find that it is easy to print to HTML, dump it on the web site, and have a similar clickable prototype to Mirror with very little work. Omnigraffle is not super cheap, but it provides the flexibility of allowing you to go very deep in the prototype if you want, layers that make building functionality quick and easy, and simple, high fidelity UI components. Happy to talk more if you want to set up a call.

The answer depends on your situation: You could be a start-up that is more concerned with not blowing tons of money, or you could be in a rush to be done. In both situations, your objective is to find inventory sources that work quickly and then massively scale into those inventory sources. I am going to assume that the goal is to be efficient. In this case, I would start with relatively few inventory pieces and a pretty low bid. The goal of your experiment is to determine what inventory DOESN'T work while spending as little money as possible. By concentrating your spend on a few pubs, you can get to significant results quickly. If you spread out your spend, you won't generate enough clicks or backend actions per inventory source quickly enough to determine if one is actually working. (In my latter user persona, you can simply spend gobs of money to get significant results on lots of inventory quickly.) So the moral of the story is that you want to get to results, you have to scale the number of pubs you work with to align with the amount of money you are planning to spend. You would like to, within a day or two, rule out bad inventory and focus on new inventory, or alternately, make massive investments in working inventory to print money for yourself. I would be happy to do a call to further discuss.