It sounds like your question is about measuring the effectiveness of your SEO efforts more than anything. In this case, number of external links really isn't going to give much away. Here are some alternative metrics:
Search traffic excluding brand name
Increases in search traffic volume is strongly and positively correlated with SEO. Because you will always rank for brand name queiries and searchers will always find your brand if they are searching for it - these searches add noise which is why we exclude them when measuring SEO effectiveness.
Keyword rankings
You may be focusing on 10 to 20 keywords on your campaign. Measure the rankings and see if the pattern is increasing. However, this metric on its own can be misleading as keyword ranking may not improve while long tail (search) traffic is sky rocketing!
Absolute conversions
The end goal. Have you seen an improvement in the number of conversions? Use Google analytics to set up goals and measure the number of goals coming from visitors that found you through search.
Apologies if I've somewhat strayed from your original question - but I feel that you should not be measuring whether or not your SEO "efforts are fruitful" by monitoring new links.
Nick