You have mentioned both paid and free marketing platform online.
With Nairaland and Instagram, you can post about your SME business there, while you have to pay to use the google adwords program.
Now, going back to your question - what other ads options are available to Nigerian SME's?
Truly, you have very limited options; but you can make good use of what you have (Biggest free promotion platform) to promote your business.
And what is this biggest business promotion platform that you have.
The answer is Google Search engine Result Pages (SERPs).
With the google result pages, your can position your business to be rank higher than that of your competitions. By doing so, you give yourself the opportunity to sell more products and service.
I cannot start explaining the various ways to take advantage of the SERPs. But you can read more on internet Marketing Strategy to help your business online.
In the article, it explained various ways to out perform your competitors online without spending too much on marketing.