Though there is no limit in general, some of these sites have their own rules. Like when I was active with Digg for a client, they had a limitation of not more than 5 posts a day. Though there was a cheat around and we could post around 10 in a day - spread out the 10 evenly and post at different time slots.
The thing to take a note is - do not confine yourself just to posting. Do other activities on the site as well - digg up or bury - behave is the word as otherwise they are surely going to ban you thinking you to be a bot or a spammer.