Hey Jonathan, nice to meet you.
App discovery is unfortunately a really tough issue without really good, easy answers today. You're not the only one facing this challenge. And it gets even tougher when you're marketing an SDK vs. an app, because there's an "SDK Fatigue" setting in among developers.
The only real long-term answer is that you have to have an SDK that provides real value to developers. They'll talk and it'll pick up over time. So focusing on product/market fit is the main thing.
Having said that, when we first started Socialize, we did some "guerrilla" things that got the ball rolling for us. Here's a list:
- 5 Minute Install: We've been app developers too, and we understand how hard it can be to get an SDK working in an app -- and if it doesn't work right away, you just move on to something else. That effort paid off with developer responses like these:

In fact, we held a $2,500 contest to see who could implement Socialize the fastest. The winner did it in just 23 seconds. We ere floored.

- Killer Documentation: We've really focused on having world-class documentation of our API and SDKs and many developers have thanked us for that. Again, it all starts with having a really great product.
- Open Source SDK: Our approach to the SDK was to make it open source with pre-built views. If someone wanted to, they could just use our UI elements and not hook the SDK into our back-end system. That's OK. Giving developers control over the UX and UI was an early, core decision for us, because we wanted to make it insanely easy to integrate Socialize into an app. We also put a lot of designer resources into making the views excellent. Here, for example, is the Comment section of our SDK documentation with the pre-built views.

- Always-on Support: This might not seem like a marketing item, but it is. We use Get Satisfaction for our support forum, and we make sure that all questions are answered in hours or days by having our developers "round robin" on support each week.
- Lots of Panels: Being a Subject Matter Expert is a great way to get publicity and give developers and the ecosystem a sense of comfort that you know what you're doing.

- Power in Numbers & Knowledge Sharing: Our developer evangelist at the time, Jeremia, created a group of developer evangelists called The Devangelists, which meets on a regular basis to trade tips on user acquisition (highly recommend you join!)
- Schwag Marketing: We created shirts and stickers and have an online form where developers can get a free shirt. Anytime a developer does something awesome, we send him or her a shirt.

- Beer Marketing: We put Socialize stickers on cans of beer and sponsored Meetups. We even bought a freezer so we could keep ice on hand so Jeremia or another employee could quickly get out to a relevant Meetup with cold beers.

- Developer Workshops: We invite developers to come in and we'll help them with the integration in our office. Pro tip: Charge for the workshops, but then offer coupons discounting it to free, so you can leverage the true value of the time you're dedicating.

- Celebrate Successes Publicly: When the first developer implemented Socialize, we got a big check made and sent him $500 plus a picture of all of us celebrating the win. To this day he's a very active and valued developer who continues to be invested in the success of Socialize.

Sponsor Hackathons: We sponsored a bunch of hackathons with prize money for developers who implemented our SDK.

Crash Other's Hackathons: This pro tip was given to us by the developer evangelist of a very well known API company. She suggested that instead of sponsoring hackathons, we just show up at other's hackathons and hand out postcards saying we'd offer a $500 prize if a developer included Socialize in their app. So that's what we did!

You can also see our Socialize photo stream to see other things we did which I haven't outlined here.
At the end of the day, it all starts with having a great product, and I have my co-founders Sean and Isaac, as well as the rest of our team to thank for that. No amount of marketing will help a poor product achieve success.
Interestingly, as we've grown, we've stopped doing many of these things because we're now focused on the Fortune1000 set, and while these tips are great for getting a critical mass going, they don't translate into getting larger branded apps on board. If you'd like me to put up a separate post about some of the things we're doing for that target audience, just let me know in the comments.