When buying traffic from any partner (e.g. Google, Bing, Yahoo, or even intext or email companies like Miva, Adconion, etc.) there is one thing you supply to them that is uniquely yours: the destination URL. Social updates, tweets, and emails that contain links to our destination sites also contain URLs that we provide as the originators of those updates we're sharing with our customers. This URL contains the parameters (if you choose to include them) that make it possible to later analyze the performance of your campaigns and adgroups in analytics. Many times, even when we think we've provided URLs with parameters to our traffic partners, or in a Tweet or Facebook share, we don't sometimes see the results in Analytics. Please let me know how often you've seen this issue, and what impact it may be causing.
It can be pretty common. It's different for different networks, some will over-report and some under-report. Google for example generally under-reports conversions, so we adjust for that on our end. Social buys generally over-report due to bots etc. It's definitely annoying but once you get your baseline metrics down you can better estimate your true costs and revenue.