No, it is not possible to select multiple features graphically in an ArcGIS Online web map.
Unlike ArcGIS for Desktop and ArcGIS Explorer Online, an ArcGIS Online web map does not have the interactive function to select multiple features simultaneously; only a single feature can be selected at a time.
However, as an alternative method to select multiple features simultaneously, select multiple records in the attribute table of the feature layer in the ArcGIS Online web map. Refer to the steps below on how to select multiple records in an ArcGIS Online web map.
1. On the My Map page in ArcGIS Online, click the Content icon. In the Content field, hover the pointer over the target layer, and click the Attribute Table icon (A). This launches the attribute table.
2. Select the desired records from the table to highlight the records on the map.
3. If the table has multiple fields and values, apply a filter (B) to select the preferred values. Refer to the following link for more information on filters, ArcGIS Online Help: Apply filters.
Please refer to the following webpage for ArcGIS Online updates, ArcGIS Online Help: What’s New.