Well depends on the stakes, number of players and how many hands you saw, but a straight/flush/full isn't definitely a seldom seen kind of hand.
- About 0.4% of hands are made straights.
- Sometimes players will draw to open ended straights or even inside straights.
- About 0.2% of hands are made flushes.
- About 1% of hands are 4-flushes, many of those may get drawn to.
- About 0.14% of hands are made full houses.
- Some players may accidentally get one trying to get pairs.
As a very inaccurate guess, I'd say around 1.5% of the time a player may get any of those hands at a showdown. Now if it's a full ring game, then any of those hands is bound to appear every few games. It's also quite possible that it was just a lucky session, but I don't think so.
Also remember that online you see lots of hands per hour and humans usually remembers the strange things and discard the rest.