Is there a downside to keeping my kid off social sites?
In most cases, no. Kids who are exposed to a variety of experiences through home and school, including digital ones, likely will pick up the technical skills they need to interact socially online whenever the time is right.
However, kids may pay a social price within their circle of friends if they're not part of the online world. Kids' online worlds and their "real" worlds are porous. Being in the know about the virtual world means they're part of the group in the "real world." Often kids will plan to meet up online after school. They talk about the games they're playing and may even enact them on the playground. In some schools, there's a certain status attained by being in a particular online world with other kids.
Your kids may not be the only ones whose parents keep them off social sites, but if the groups of friends they want to hang with are all on, they may not feel as involved. Only you can determine how important this is to your kids and how not being on social sites affects them.