Welcome to the Q&A Forum

Hey there, best of luck in your search for a firm to help you with link building.  I can't tell you who to pick, but for a small to medium sized business 2500 a month is MORE than fair, but you should be able to nab a pretty strong link builder.

A couple of tips for you as you seek out the right firm:

• Make sure they are truthful about who is building your links, check out profiles of their team on Linkedin (don't tell them you are going to do this).
• A TRUE link builder will ask you a lot of questions about what makes your business unique, what innovative things you have done, who is going to write your copy, etc.  They should be asking you to build assets that will naturally attract links.
• They should try to build relationships with trade pubs, find out what kind of content those pubs and bloggers are seeking and have you create it
• Don't think that if you pay someone 2500/mo that they can get you the best type and highest quality links w/o your involvement.  I think that is what gets a lot of linkbuilders into trouble in the first place, building links on any old site to just "get links"

Tag Crowd, sorry about the mumbling

Trung beat me to it but +1 to keywordtool.io, an often overlooked thing is to ask your users how they would find a solution to their issue - with a tool like seed keywords - http://seedkeywords.com/.

We have recently had success with looking at the old pre-not provided data for long tail queries, the data is fresh enough that it might open your eyes a bit to new opps and a lot of people aren't doing that.  One of our peeps Amanda might be publishing something soon about this, it helped us build / refresh content better than if we had used other tools.

What kind of ecommerce business?  I ask because I have a client who in the next 2-3 years is projected to make more from pinterest than natural search (assuming unbranded).  Maybe given your audience FB isn't the best place? Just thinking out loud here.