There is no specific figure for conversion rates, if that's what you're talking about. I've run a few campaigns on social networks, and got CTR between 1/700 and 1/8, with visitor-to-client conversion from 1/100 to 1/3 (non-paying clients though). In general any conversion rate can be anywhere between 0 and 100%.
It highly depends on the product itself, the marketing message, targeting, platform, etc. The only way to know is to try - there is no real alternative to an experiment here. Fortunately, running a trial campaign is very easy and cheap these days. Also, you can increase conversion rates significantly, but that takes time, hard work, and creativity.
If, however, for some reason you need a number urgently, then assume a CTR of 1% and visitor-to-paying client of 1%. In my experience, that's a reasonable order of magnitude "guestimate". I think Facebook charges something like 20 cents for a 1% CTR, so that would be 20 dollars per paying customer. The average Life-Time-Value of such customer would probably have to be over 60 dollars to make the business model work.