Find apps that are B2B and go to the review sections. Specifically going to Google's play or Chrome store and look at reviews and follow them to Google+ to engage.
I took that same angle for B2C app I was building last year.
Check it out. http://blendah.com/post/37331434653/lean-startup-hack
If you know exactly who your customer is, Linkedin is a great option because you'll be able to target by job title or by company.
When you do reach out, try to be more personal and less salesy. The relationship should always seem mutually beneficial.
Best way in the beginning is to do it the old fashioned way: calls and emails to set up demos.
First, I use manta.com. You can search by different industries and create a list
Then email and call.
Approach the email/call in a way that you want their input on a new tool, and frame it that you are researching their industry to get feedback. You should be able to get demos from that.
Then when you are on a demo, make sure you have a really attractive offer to join the beta program (you should call this the "Founding Member Program"). For example, if they join, they get free access to the beta product then 10% off for life when you launch the full product.
Let me know if I can expand or help any more.
There isn't a single B2B category you could be trying to sell to that doesn't have at least a handful of active Twitter users, bloggers and enthusiastic early adopters who would be thrilled to give your product a try.
A simple place to start is Twitter. Use key industry search terms and reach-out to the people who have the most followers (but who are not followed by as many people), who have - in your own opinion - interesting things to say & share, and who actively engage in @discussions.
There are some people who just tweet out links but are not engaged in conversation on Twitter.
Once you've found these people, start following them and wait a couple of days at least. The best case scenario is to build a relationship with them by replying to and /or retweeting their links first.
Whenever you're ready to ask, be complimentary, briefly describe your ask and include a link with more information. Don't do many asks all at the same time, as it might be perceived to be spamming.
There are additional resources to cover but I've found Twitter to be an incredibly effective place to recruit users for a broad variety of applications. Happy to talk to you about this further, if you'd like.