The answer to this depends on what aspect of the definition of a social movement you think is most important. There are many ways in which the Tea Party is a social movement, but there is one very important way in which it is not.
One aspect of social movements is that they are generally a bottom-up phenomenon. The Tea Party qualifies on this score since it was not a creation of the political leaders of the country. Instead, it arose spontaneously from the grassroots.
A second aspect of social movements is that they arise among people who have a shared sense of grievance. The Tea Party qualifies on this score since the people who make it up generally feel that the government has abandoned them and their values. They are very angry about the way that the country is going.
Another aspect of social movements is that they are generally used by political outsiders. This is more ambiguous. Tea Party members would generally characterize themselves in this way because they are not typically upper-class people. However, it is hard to say that middle class white people are political outsiders in America. It is also hard to argue that any group that has presidential candidates vying for its approval is made up of outsiders.
This brings us to the last aspect of social movements, the fact that they tend not to use conventional tactics to advance their agenda. Here, the Tea Party clearly does not fit the definition. The major Tea Party tactics have been electoral, which is one of the most mainstream of political actions. This is not a protest movement in the vein of the Civil Rights Movement.
Overall, then, I would argue that the Tea Party is not a social movement because its members are not enough of outsiders and because its major tactic is centered around getting people to vote.
A tea party is a social movement due to the fact that is has become a major contributor to how people feel about taxes in their everyday life. Also, there are hundreds of thousands of people involved, making in a major social movement in our country.