PR is your best bet and it's pretty easy. I suggest baby steps.

1) Who are your early adopters? Age, sex, location, profession, hobbies, what do they read. Survey monkey and offer a free month or something

2) Choose a segment of your 50.

3) Create a list of blogs, newspapers, magazines to pitch that the segment reads

4) Begin pitching the outlets.

5) Measure the new sign ups per article using google analytics

There's alot of small details in each step but this is the big picture