http://www.reddit.com/r/TheWayWeWere/comments/23b2a5/on_april_17_1907_the_ellis_island_immigration/ i learned this fact from a post in r/thewaywewere
I read a pdf. for a project I'm working on. It concerns Jewish immigration to the UK but a lot of those arriving were just passing through on their way to the US. It's an interesting read and contains several tables showing the numbers arriving in the UK. Hope it's of some interest.
Assuming the numbers for 1910 are not far off from those of 1907 - the largest grouping of immigrants (64%) came from either Austria-Hungary (25%), Italy (21%), or Russia (18%).
Austria-Hungary was chock full of competing nationalities during this period. A large portion of the Polish population came over here (the Polish represented 13% of the total by themselves, though their figure should be divided between Austria, Germany and Russia). A large segment of the Austrian number was from Croatia and Slovenia - which points to the ethnic turmoil that went on with Hungary's activities in the Balkan region, and general resistance to their harsh process of Magyarization. The resistance to the Magyars also prompted many Slovakians to emigrate, while Ruthenians were more or less convinced to leave due to oppression by the Poles.
Add the deteriorating economic situation of the late Austro-Hungarian empire, and it's a good recipe for large numbers of people to emigrate. The same thing could potentially be said about Italy - poor economic conditions led many to look for a different life. Someone else would have to comment upon whether or not there's any social significance to the fact that more than 86% of those who came from Italy were classified as Southern Italians. I know the south has traditionally had less wealth than the Piedmont region, I just don't know how well they were enfranchised under the national government.
Regarding Russia - Nicholas II hated non-Russians and repressed them. It's notable that Stalin became a socialist organizer a few years before this date, so Russia was certainly not in the best of economic straits. 1905 would be pivotal in that the Russians were defeated in the Russo-Japanese War, and domestically the country was gripped by what Lenin called the Great Rehearsal - a revolution of the proletariat. In the Caucasus it took on a racial overtone with conflict between Armenians and Tatars. One element of the propaganda was that foreign jewish elements were stealing the wealth of the people, especially in Baku - so that might account for the fact that 8% of those who came over in 1910 were classified as Hebrew.
The revolution ended in the same year it started when Nicholas conceded to a constitutional monarchy with an elected Duma. However, that doesn't mean Russia's social and economic problems were over. The discontent remained simmering under the surface for another 12 years before it finally blew over and toppled the Russian state - and certainly many people would continue to look for a way out in the meantime.
(Edit: And, I do believe the numbers dropped off after 1907 because that was when the United States started to seriously consider immigration quotas. Another factor which seems to have later dried up the flood was the end of WWI, where emigration from the United States to southern and eastern Europe actually exceeded the number of immigrants coming this way. My guess is that people felt there was new hope in the homeland with the death of the Austro-Hungarian Empire. By 1920, most of the immigrants were coming from Canada or Mexico)