What you should always ask, immediately after reading "insiders have sold $X of shares" is "out of how much, total?".
Also, like a sibling comment points out, may of these people have predetermined plans to exercise and sell, and may have tax reasons for selling 30% of stock awards (https://www.investopedia.com/terms/r/rule-10b5-1.asp).
"Thing that has been planned for years happens on schedule, with no relation to current events whatsoever. We're writing about it solely because doing so allows us to write a headline that misleadingly implies it has some relation to current events."
It's been 6 months? How many RSUs does facebook give out? Conventional wisdom with RSUs is you sell the moment they vest. Why hold on to a security you cannot actively trade, and why hold security in the company that also handles your paycheck -- it's putting all your eggs in one basket.
This doesn't really mean anything. It's almost all part of the stock plans that were put in place well before the scandal broke.
Even barring that, the data is meaningless unless you show the insider sales from before the scandal for comparison.
For all we know, insider sales might have gone down since the scandal.
Bloomberg usually has better reporting than this.