It looks like Twitter API restricts number of returned tweets. You should check this out in the API documentation. Keeping the restriction in mind, you can use the since and sinceID arguments of searchTwitter() within a loop, something like:
for (i in 1:20) {
if (i==1) search = searchTwitter('facebook',cainfo="cacert.pem",n=2, since='2014-04-15')
else search = searchTwitter('facebook',cainfo="cacert.pem",n=2, since='2014-04-15', sinceID=search[[1]]$id)
print(search)
Sys.sleep(10)
}
You may need to adjust the Sys.sleep(10) portion if you hit API restrictions.